diff --git a/src/mol-util/computation.ts b/src/mol-util/computation.ts
index 6bafb7a99b7159cc6fa0edc65554b86ab32e8458..81f92920a3cf9587295c00f107f7ff3e010a20f4 100644
--- a/src/mol-util/computation.ts
+++ b/src/mol-util/computation.ts
@@ -124,8 +124,6 @@ class ObservableContext implements Computation.Context {
     private observers: Computation.ProgressObserver[] | undefined = void 0;
     private progress: Computation.Progress = { message: 'Working...', current: 0, max: 0, elapsedMs: 0, isIndeterminate: true, requestAbort: void 0 };
 
-    lastDelta = 0;
-
     private checkAborted() {
         if (this.abortRequested) throw Computation.Aborted;
     }
@@ -186,7 +184,6 @@ class ObservableContext implements Computation.Context {
             }
         }
 
-        this.lastDelta = time - this.lastUpdated;
         this.lastUpdated = time;
 
         return Scheduler.immediatePromise();
@@ -223,11 +220,10 @@ class ChunkerImpl implements Computation.Chunker {
     private processedSinceUpdate = 0;
     private updater: Computation.Context['update'];
 
-    private computeChunkSize() {
-        const lastDelta = (this.context as ObservableContext).lastDelta || 0;
-        if (!lastDelta) return this.nextChunkSize;
+    private computeChunkSize(delta: number) {
+        if (!delta) return this.nextChunkSize;
         const rate = (this.context as ObservableContext).updateRate || DefaulUpdateRateMs;
-        const ret = Math.round(this.processedSinceUpdate * rate / lastDelta + 1);
+        const ret = Math.round(this.processedSinceUpdate * rate / delta + 1);
         this.processedSinceUpdate = 0;
         return ret;
     }
@@ -246,17 +242,22 @@ class ChunkerImpl implements Computation.Chunker {
     async process(nextChunk: (size: number) => number, update: (updater: Computation.Context['update']) => Promise<void> | void, nextChunkSize?: number) {
         if (typeof nextChunkSize !== 'undefined') this.setNextChunkSize(nextChunkSize);
 
+        // track time for the actual computation and exclude the "update time"
+        let chunkStart = Computation.now();
         let lastChunkSize: number;
         while ((lastChunkSize = nextChunk(this.getNextChunkSize())) > 0) {
             this.processedSinceUpdate += lastChunkSize;
             if (this.context.requiresUpdate) {
+                let time = Computation.now();
                 await update(this.updater);
-                this.nextChunkSize = this.computeChunkSize();
+                this.nextChunkSize = this.computeChunkSize(time - chunkStart);
+                chunkStart = Computation.now();
             }
         }
         if (this.context.requiresUpdate) {
+            let time = Computation.now();
             await update(this.updater);
-            this.nextChunkSize = this.computeChunkSize();
+            this.nextChunkSize = this.computeChunkSize(time - chunkStart);
         }
     }
 
diff --git a/src/perf-tests/structure.ts b/src/perf-tests/structure.ts
index 6fdd03c754e5fbd9a87e892dd9fab97e891b3430..013f08ae0a1642a97a00f4c734034f70d4ade73b 100644
--- a/src/perf-tests/structure.ts
+++ b/src/perf-tests/structure.ts
@@ -29,6 +29,23 @@ async function readData(path: string) {
     }
 }
 
+function *test() {
+    yield 10;
+    return 15;
+}
+
+async function runIt<T>(itP: () => IterableIterator<T>) {
+    const it = itP();
+    let lastValue: T | undefined;
+    while(true) {
+        const { value, done } = it.next();
+        if (done) return value;
+        lastValue = value;
+    }
+}
+
+runIt(test).then(r => console.log('rerdasdasda', r))
+
 export async function readCIF(path: string) {
     console.time('readData');
     const input = await readData(path)
diff --git a/src/script.ts b/src/script.ts
index 03c5d970ae0e3540eecf67b85967d1af0ecad3fa..03ac84b99889c9eef8fb0bc8283f4d47fe3e9a50 100644
--- a/src/script.ts
+++ b/src/script.ts
@@ -148,13 +148,14 @@ async function runCIF(input: string | Uint8Array) {
 export async function _cif() {
     let path = `./examples/1grm_updated.cif`;
     // path = '../test/3j3q.cif'  // lets have a relative path for big test files
+    //path = 'e:/test/quick/3j3q_updated.cif';
     const input = await readFileAsync(path, 'utf8')
     console.log('------------------');
     console.log('Text CIF:');
     runCIF(input);
 
     path = `./examples/1cbs_full.bcif`;
-    // const path = 'c:/test/quick/3j3q.cif';
+    
     const input2 = await readFileAsync(path)
     console.log('------------------');
     console.log('BinaryCIF:');