diff --git a/packages/kbn-optimizer/src/common/event_stream_helpers.test.ts b/packages/kbn-optimizer/src/common/event_stream_helpers.test.ts
index 60982abff2d8..f6f684153279 100644
--- a/packages/kbn-optimizer/src/common/event_stream_helpers.test.ts
+++ b/packages/kbn-optimizer/src/common/event_stream_helpers.test.ts
@@ -18,52 +18,221 @@
  */
 
 import * as Rx from 'rxjs';
-import { toArray } from 'rxjs/operators';
+import { toArray, take } from 'rxjs/operators';
 
-import { summarizeEvent$ } from './event_stream_helpers';
+import { summarizeEventStream } from './event_stream_helpers';
 
-it('emits each state with each event, ignoring events when reducer returns undefined', async () => {
-  const values = await summarizeEvent$(
-    Rx.of(1, 2, 3, 4, 5),
-    {
-      sum: 0,
-    },
-    (state, event) => {
-      if (event % 2) {
-        return {
-          sum: state.sum + event,
-        };
-      }
+it('emits each state with each event, ignoring events when summarizer returns undefined', async () => {
+  const event$ = Rx.of(1, 2, 3, 4, 5);
+  const initial = 0;
+  const values = await summarizeEventStream(event$, initial, (state, event) => {
+    if (event % 2) {
+      return state + event;
     }
-  )
+  })
     .pipe(toArray())
     .toPromise();
 
   expect(values).toMatchInlineSnapshot(`
     Array [
       Object {
-        "state": Object {
-          "sum": 0,
-        },
+        "state": 0,
       },
       Object {
         "event": 1,
-        "state": Object {
-          "sum": 1,
-        },
+        "state": 1,
       },
       Object {
         "event": 3,
-        "state": Object {
-          "sum": 4,
-        },
+        "state": 4,
       },
       Object {
         "event": 5,
-        "state": Object {
-          "sum": 9,
-        },
+        "state": 9,
       },
     ]
   `);
 });
+
+it('interleaves injected events when source is synchronous', async () => {
+  const event$ = Rx.of(1, 7);
+  const initial = 0;
+  const values = await summarizeEventStream(event$, initial, (state, event, injectEvent) => {
+    if (event < 5) {
+      injectEvent(event + 2);
+    }
+
+    return state + event;
+  })
+    .pipe(toArray())
+    .toPromise();
+
+  expect(values).toMatchInlineSnapshot(`
+    Array [
+      Object {
+        "state": 0,
+      },
+      Object {
+        "event": 1,
+        "state": 1,
+      },
+      Object {
+        "event": 3,
+        "state": 4,
+      },
+      Object {
+        "event": 5,
+        "state": 9,
+      },
+      Object {
+        "event": 7,
+        "state": 16,
+      },
+    ]
+  `);
+});
+
+it('interleaves injected events when source is asynchronous', async () => {
+  const event$ = Rx.of(1, 7, Rx.asyncScheduler);
+  const initial = 0;
+  const values = await summarizeEventStream(event$, initial, (state, event, injectEvent) => {
+    if (event < 5) {
+      injectEvent(event + 2);
+    }
+
+    return state + event;
+  })
+    .pipe(toArray())
+    .toPromise();
+
+  expect(values).toMatchInlineSnapshot(`
+    Array [
+      Object {
+        "state": 0,
+      },
+      Object {
+        "event": 1,
+        "state": 1,
+      },
+      Object {
+        "event": 3,
+        "state": 4,
+      },
+      Object {
+        "event": 5,
+        "state": 9,
+      },
+      Object {
+        "event": 7,
+        "state": 16,
+      },
+    ]
+  `);
+});
+
+it('interleaves mulitple injected events in order', async () => {
+  const event$ = Rx.of(1);
+  const initial = 0;
+  const values = await summarizeEventStream(event$, initial, (state, event, injectEvent) => {
+    if (event < 10) {
+      injectEvent(10);
+      injectEvent(20);
+      injectEvent(30);
+    }
+
+    return state + event;
+  })
+    .pipe(toArray())
+    .toPromise();
+
+  expect(values).toMatchInlineSnapshot(`
+    Array [
+      Object {
+        "state": 0,
+      },
+      Object {
+        "event": 1,
+        "state": 1,
+      },
+      Object {
+        "event": 10,
+        "state": 11,
+      },
+      Object {
+        "event": 20,
+        "state": 31,
+      },
+      Object {
+        "event": 30,
+        "state": 61,
+      },
+    ]
+  `);
+});
+
+it('stops an infinite stream when unsubscribed', async () => {
+  const event$ = Rx.of(1);
+  const initial = 0;
+  const summarize = jest.fn((prev, event, injectEvent) => {
+    // always inject a follow up event, making this infinite and synchronous
+    injectEvent(event + 1);
+    return prev + event;
+  });
+
+  const values = await summarizeEventStream(event$, initial, summarize)
+    .pipe(take(11), toArray())
+    .toPromise();
+
+  expect(values).toMatchInlineSnapshot(`
+    Array [
+      Object {
+        "state": 0,
+      },
+      Object {
+        "event": 1,
+        "state": 1,
+      },
+      Object {
+        "event": 2,
+        "state": 3,
+      },
+      Object {
+        "event": 3,
+        "state": 6,
+      },
+      Object {
+        "event": 4,
+        "state": 10,
+      },
+      Object {
+        "event": 5,
+        "state": 15,
+      },
+      Object {
+        "event": 6,
+        "state": 21,
+      },
+      Object {
+        "event": 7,
+        "state": 28,
+      },
+      Object {
+        "event": 8,
+        "state": 36,
+      },
+      Object {
+        "event": 9,
+        "state": 45,
+      },
+      Object {
+        "event": 10,
+        "state": 55,
+      },
+    ]
+  `);
+
+  // ensure summarizer still only called 10 times after a timeout
+  expect(summarize).toHaveBeenCalledTimes(10);
+  await new Promise(resolve => setTimeout(resolve, 1000));
+  expect(summarize).toHaveBeenCalledTimes(10);
+});
diff --git a/packages/kbn-optimizer/src/common/event_stream_helpers.ts b/packages/kbn-optimizer/src/common/event_stream_helpers.ts
index c1585f79ede6..d07af32f8889 100644
--- a/packages/kbn-optimizer/src/common/event_stream_helpers.ts
+++ b/packages/kbn-optimizer/src/common/event_stream_helpers.ts
@@ -18,39 +18,91 @@
  */
 
 import * as Rx from 'rxjs';
-import { scan, distinctUntilChanged, startWith } from 'rxjs/operators';
 
 export interface Update<Event, State> {
   event?: Event;
   state: State;
 }
 
-export type Summarizer<Event, State> = (prev: State, event: Event) => State | undefined;
+export type EventInjector<Event> = (event: Event) => void;
+export type Summarizer<Event, State> = (
+  prev: State,
+  event: Event,
+  injectEvent: EventInjector<Event>
+) => State | undefined;
 
 /**
  * Transform an event stream into a state update stream which emits
  * the events and individual states for each event.
  */
-export const summarizeEvent$ = <Event, State>(
+export const summarizeEventStream = <Event, State>(
   event$: Rx.Observable<Event>,
   initialState: State,
-  reducer: Summarizer<Event, State>
+  summarize: Summarizer<Event, State>
 ) => {
-  const initUpdate: Update<Event, State> = {
-    state: initialState,
-  };
+  return new Rx.Observable<Update<Event, State>>(subscriber => {
+    const eventBuffer: Event[] = [];
 
-  return event$.pipe(
-    scan((prev, event): Update<Event, State> => {
-      const newState = reducer(prev.state, event);
-      return newState === undefined
-        ? prev
-        : {
+    let processingEventBuffer = false;
+    let eventStreamComplete = false;
+    let previousState = initialState;
+
+    const injectEvent = (nextEvent: Event) => {
+      eventBuffer.push(nextEvent);
+
+      if (processingEventBuffer) {
+        return;
+      }
+
+      try {
+        processingEventBuffer = true;
+
+        while (eventBuffer.length && !subscriber.closed) {
+          const event = eventBuffer.shift()!;
+          const nextState = summarize(previousState, event, injectEvent);
+
+          if (nextState === undefined) {
+            // skip this event
+            continue;
+          }
+
+          // emit state update
+          previousState = nextState;
+          subscriber.next({
             event,
-            state: newState,
-          };
-    }, initUpdate),
-    distinctUntilChanged(),
-    startWith(initUpdate)
-  );
+            state: nextState,
+          });
+        }
+
+        if (eventStreamComplete) {
+          subscriber.complete();
+        }
+      } catch (error) {
+        subscriber.error(error);
+      } finally {
+        processingEventBuffer = false;
+      }
+    };
+
+    // send initial "update"
+    subscriber.next({
+      state: initialState,
+    });
+
+    // inject all subsequent events to the internal eventBuffer
+    subscriber.add(
+      event$.subscribe(
+        injectEvent,
+        error => {
+          subscriber.error(error);
+        },
+        () => {
+          eventStreamComplete = true;
+          if (!processingEventBuffer && eventBuffer.length === 0) {
+            subscriber.complete();
+          }
+        }
+      )
+    );
+  });
 };
diff --git a/packages/kbn-optimizer/src/integration_tests/basic_optimization.test.ts b/packages/kbn-optimizer/src/integration_tests/basic_optimization.test.ts
index b35788009dd9..fec31cbe40df 100644
--- a/packages/kbn-optimizer/src/integration_tests/basic_optimization.test.ts
+++ b/packages/kbn-optimizer/src/integration_tests/basic_optimization.test.ts
@@ -179,6 +179,7 @@ it('uses cache on second run and exist cleanly', async () => {
       "initializing",
       "initializing",
       "initialized",
+      "success",
     ]
   `);
 });
diff --git a/packages/kbn-optimizer/src/log_optimizer_state.ts b/packages/kbn-optimizer/src/log_optimizer_state.ts
index 1ee4e47bfd9e..5217581d1b41 100644
--- a/packages/kbn-optimizer/src/log_optimizer_state.ts
+++ b/packages/kbn-optimizer/src/log_optimizer_state.ts
@@ -77,10 +77,6 @@ export function logOptimizerState(log: ToolingLog, config: OptimizerConfig) {
             loggedInit = true;
             log.info(`initialized, ${state.offlineBundles.length} bundles cached`);
           }
-
-          if (state.onlineBundles.length === 0) {
-            log.success(`all bundles cached, success after ${state.durSec}`);
-          }
           return;
         }
 
@@ -123,10 +119,16 @@ export function logOptimizerState(log: ToolingLog, config: OptimizerConfig) {
         if (state.phase === 'success') {
           const buildCount = bundlesThatWereBuilt.size;
           bundlesThatWereBuilt.clear();
-          log.success(
-            `${buildCount} bundles compiled successfully after ${state.durSec} sec` +
-              (config.watch ? ', watching for changes' : '')
-          );
+
+          if (state.offlineBundles.length && buildCount === 0) {
+            log.success(`all bundles cached, success after ${state.durSec} sec`);
+          } else {
+            log.success(
+              `${buildCount} bundles compiled successfully after ${state.durSec} sec` +
+                (config.watch ? ', watching for changes' : '')
+            );
+          }
+
           return true;
         }
 
diff --git a/packages/kbn-optimizer/src/optimizer/handle_optimizer_completion.test.ts b/packages/kbn-optimizer/src/optimizer/handle_optimizer_completion.test.ts
index 7a8575a6c91f..3cc58e744a7b 100644
--- a/packages/kbn-optimizer/src/optimizer/handle_optimizer_completion.test.ts
+++ b/packages/kbn-optimizer/src/optimizer/handle_optimizer_completion.test.ts
@@ -22,7 +22,7 @@ import { REPO_ROOT } from '@kbn/dev-utils';
 
 import { Update } from '../common';
 
-import { OptimizerState } from './optimizer_reducer';
+import { OptimizerState } from './optimizer_state';
 import { OptimizerConfig } from './optimizer_config';
 import { handleOptimizerCompletion } from './handle_optimizer_completion';
 import { toArray } from 'rxjs/operators';
diff --git a/packages/kbn-optimizer/src/optimizer/handle_optimizer_completion.ts b/packages/kbn-optimizer/src/optimizer/handle_optimizer_completion.ts
index 9587902cc418..5dd500cd7a9e 100644
--- a/packages/kbn-optimizer/src/optimizer/handle_optimizer_completion.ts
+++ b/packages/kbn-optimizer/src/optimizer/handle_optimizer_completion.ts
@@ -23,7 +23,7 @@ import { createFailError } from '@kbn/dev-utils';
 
 import { pipeClosure, Update } from '../common';
 
-import { OptimizerState } from './optimizer_reducer';
+import { OptimizerState } from './optimizer_state';
 import { OptimizerConfig } from './optimizer_config';
 
 export function handleOptimizerCompletion(config: OptimizerConfig) {
@@ -44,11 +44,6 @@ export function handleOptimizerCompletion(config: OptimizerConfig) {
             return;
           }
 
-          if (prevState?.phase === 'initialized' && prevState.onlineBundles.length === 0) {
-            // all bundles cached
-            return;
-          }
-
           if (prevState?.phase === 'issue') {
             throw createFailError('webpack issue');
           }
diff --git a/packages/kbn-optimizer/src/optimizer/index.ts b/packages/kbn-optimizer/src/optimizer/index.ts
index 3df8ed930266..84fd395e9897 100644
--- a/packages/kbn-optimizer/src/optimizer/index.ts
+++ b/packages/kbn-optimizer/src/optimizer/index.ts
@@ -19,7 +19,7 @@
 
 export * from './optimizer_config';
 export { WorkerStdio } from './observe_worker';
-export * from './optimizer_reducer';
+export * from './optimizer_state';
 export * from './cache_keys';
 export * from './watch_bundles_for_changes';
 export * from './run_workers';
diff --git a/packages/kbn-optimizer/src/optimizer/optimizer_reducer.ts b/packages/kbn-optimizer/src/optimizer/optimizer_state.ts
similarity index 91%
rename from packages/kbn-optimizer/src/optimizer/optimizer_reducer.ts
rename to packages/kbn-optimizer/src/optimizer/optimizer_state.ts
index c1e6572bd7e7..ac2a9b8ce1f8 100644
--- a/packages/kbn-optimizer/src/optimizer/optimizer_reducer.ts
+++ b/packages/kbn-optimizer/src/optimizer/optimizer_state.ts
@@ -30,8 +30,13 @@ export interface OptimizerInitializedEvent {
   type: 'optimizer initialized';
 }
 
+export interface AllBundlesCachedEvent {
+  type: 'all bundles cached';
+}
+
 export type OptimizerEvent =
   | OptimizerInitializedEvent
+  | AllBundlesCachedEvent
   | ChangeEvent
   | WorkerMsg
   | WorkerStatus
@@ -92,16 +97,28 @@ function getStatePhase(states: CompilerMsg[]) {
   throw new Error(`unable to summarize bundle states: ${JSON.stringify(states)}`);
 }
 
-export function createOptimizerReducer(
+export function createOptimizerStateSummarizer(
   config: OptimizerConfig
 ): Summarizer<OptimizerEvent, OptimizerState> {
-  return (state, event) => {
+  return (state, event, injectEvent) => {
     if (event.type === 'optimizer initialized') {
+      if (state.onlineBundles.length === 0) {
+        injectEvent({
+          type: 'all bundles cached',
+        });
+      }
+
       return createOptimizerState(state, {
         phase: 'initialized',
       });
     }
 
+    if (event.type === 'all bundles cached') {
+      return createOptimizerState(state, {
+        phase: 'success',
+      });
+    }
+
     if (event.type === 'worker error' || event.type === 'compiler error') {
       // unrecoverable error states
       const error = new Error(event.errorMsg);
diff --git a/packages/kbn-optimizer/src/run_optimizer.ts b/packages/kbn-optimizer/src/run_optimizer.ts
index d2daa79feab7..ab12cc679bc6 100644
--- a/packages/kbn-optimizer/src/run_optimizer.ts
+++ b/packages/kbn-optimizer/src/run_optimizer.ts
@@ -20,7 +20,7 @@
 import * as Rx from 'rxjs';
 import { mergeMap, share, observeOn } from 'rxjs/operators';
 
-import { summarizeEvent$, Update } from './common';
+import { summarizeEventStream, Update } from './common';
 
 import {
   OptimizerConfig,
@@ -31,7 +31,7 @@ import {
   watchBundlesForChanges$,
   runWorkers,
   OptimizerInitializedEvent,
-  createOptimizerReducer,
+  createOptimizerStateSummarizer,
   handleOptimizerCompletion,
 } from './optimizer';
 
@@ -66,7 +66,7 @@ export function runOptimizer(config: OptimizerConfig) {
       const workerEvent$ = runWorkers(config, cacheKey, bundleCacheEvent$, changeEvent$);
 
       // create the stream that summarized all the events into specific states
-      return summarizeEvent$<OptimizerEvent, OptimizerState>(
+      return summarizeEventStream<OptimizerEvent, OptimizerState>(
         Rx.merge(init$, changeEvent$, workerEvent$),
         {
           phase: 'initializing',
@@ -76,7 +76,7 @@ export function runOptimizer(config: OptimizerConfig) {
           startTime,
           durSec: 0,
         },
-        createOptimizerReducer(config)
+        createOptimizerStateSummarizer(config)
       );
     }),
     handleOptimizerCompletion(config)