@@ -5,8 +5,6 @@ const {
   ObjectDefineProperties,
   ObjectSetPrototypeOf,
   PromisePrototypeThen,
-  PromiseReject,
-  PromiseResolve,
   PromiseWithResolvers,
   Symbol,
   SymbolToStringTag,
@@ -46,13 +44,12 @@ const {
 
 const {
   createPromiseCallback1Param,
-  createRawCallback2Params,
+  createPromiseCallback2Params,
   customInspect,
   extractHighWaterMark,
   extractSizeAlgorithm,
   getNonWritablePropertyDescriptor,
   isBrandCheck,
-  kParkedAlgorithmResult,
   kResolvedPromise,
   kState,
   kType,
@@ -262,12 +259,7 @@ class TransformStreamState {
   writable = undefined;
   controller = undefined;
   backpressure = undefined;
-  // Continuation slots replacing the spec's
-  // [[backpressureChangePromise]]; see transformStreamSetBackpressure.
-  pullPending = false;
-  pendingWrite = undefined;
-  pendingWriteChunk = undefined;
-  writeContinuation = undefined;
+  backpressureChange = undefined;
 }
 ObjectSetPrototypeOf(TransformStreamState.prototype, null);
 
@@ -374,9 +366,7 @@ const isTransformStream =
 const isTransformStreamDefaultController =
   isBrandCheck('TransformStreamDefaultController');
 
-// Raw callback (see createRawCallback*): invoked inside the try/catch of
-// transformStreamDefaultControllerPerformTransform.
-function defaultTransformAlgorithm(chunk, controller) {
+async function defaultTransformAlgorithm(chunk, controller) {
   transformStreamDefaultControllerEnqueue(controller, chunk);
 }
 
@@ -452,30 +442,24 @@ function transformStreamUnblockWrite(stream) {
 // The spec's [[backpressureChangePromise]] is only ever observed by the
 // source pull algorithm (settles when backpressure next becomes true) and
 // by a sink write arriving while backpressure is set (settles when
-// backpressure next becomes false). Both observers are internal, so the
-// promise record is replaced by continuation slots: a parked pull is
-// completed by delivering the readable controller's pull-fulfilled step,
-// and a parked write by the cached write continuation (see
-// transformStreamDefaultSinkWriteAlgorithm). Each is enqueued on the
-// shared resolved promise at the exact microtask position the old
-// record's reaction would have had.
+// backpressure next becomes false). Instead of allocating a fresh promise
+// record on every flip, the record is materialized lazily on first
+// observation and dropped once settled; flips nobody is waiting on
+// allocate nothing.
+function transformStreamBackpressureChangePromise(stream) {
+  const state = stream[kState];
+  return (state.backpressureChange ??= PromiseWithResolvers()).promise;
+}
+
 function transformStreamSetBackpressure(stream, backpressure) {
   const state = stream[kState];
   assert(state.backpressure !== backpressure);
-  state.backpressure = backpressure;
-  if (backpressure) {
-    if (state.pullPending) {
-      state.pullPending = false;
-      // The pull-fulfilled step exists: a pull parked it (see
-      // transformStreamDefaultSourcePullAlgorithm), and the readable
-      // controller creates it before invoking the pull algorithm.
-      PromisePrototypeThen(
-        kResolvedPromise,
-        state.readable[kState].controller[kState].pullFulfilled);
-    }
-  } else if (state.pendingWrite !== undefined) {
-    PromisePrototypeThen(kResolvedPromise, state.writeContinuation);
+  const backpressureChange = state.backpressureChange;
+  if (backpressureChange !== undefined) {
+    state.backpressureChange = undefined;
+    backpressureChange.resolve();
   }
+  state.backpressure = backpressure;
 }
 
 function setupTransformStreamDefaultController(
@@ -503,7 +487,7 @@ function setupTransformStreamDefaultControllerFromTransformer(
   const flush = transformer?.flush;
   const cancel = transformer?.cancel;
   const transformAlgorithm = transform ?
-    createRawCallback2Params('transformer.transform', transform, transformer) :
+    createPromiseCallback2Params('transformer.transform', transform, transformer) :
     defaultTransformAlgorithm;
   const flushAlgorithm = flush ?
     createPromiseCallback1Param('transformer.flush', flush, transformer) :
@@ -556,40 +540,18 @@ function transformStreamDefaultControllerError(controller, error) {
   transformStreamError(controller[kState].stream, error);
 }
 
-// Mirrors the reference implementation's
-// `promiseCall(transformAlgorithm, ...).then(undefined, rejectionSteps)`:
-// the returned promise settles one microtask after the (coerced) result
-// does, and a rejection errors the transform stream before propagating.
-// The raw transform callback plus the shared resolved promise for
-// non-thenable results replace the previous async wrapper's two implicit
-// promises per chunk.
-function transformStreamDefaultControllerPerformTransform(controller, chunk) {
-  const controllerState = controller[kState];
-  const transformAlgorithm = controllerState.transformAlgorithm;
-  if (transformAlgorithm === undefined) {
-    // Algorithms were cleared by a concurrent cancel/abort/close.
-    return kResolvedPromise;
-  }
-  let result;
+async function transformStreamDefaultControllerPerformTransform(controller, chunk) {
   try {
-    result = transformAlgorithm(chunk, controller);
+    const transformAlgorithm = controller[kState].transformAlgorithm;
+    if (transformAlgorithm === undefined) {
+      // Algorithms were cleared by a concurrent cancel/abort/close.
+      return;
+    }
+    return await transformAlgorithm(chunk, controller);
   } catch (error) {
-    result = PromiseReject(error);
-  }
-  if (result === null ||
-      (typeof result !== 'object' && typeof result !== 'function')) {
-    result = kResolvedPromise;
-  } else {
-    result = PromiseResolve(result);
-  }
-  controllerState.performTransformRejected ??= (error) => {
     transformStreamError(controller[kState].stream, error);
     throw error;
-  };
-  return PromisePrototypeThen(
-    result,
-    undefined,
-    controllerState.performTransformRejected);
+  }
 }
 
 function transformStreamDefaultControllerTerminate(controller) {
@@ -610,42 +572,26 @@ function transformStreamDefaultControllerTerminate(controller) {
 }
 
 function transformStreamDefaultSinkWriteAlgorithm(stream, chunk) {
-  const state = stream[kState];
   const {
     writable,
     controller,
-  } = state;
+  } = stream[kState];
   assert(writable[kState].state === 'writable');
-  if (state.backpressure) {
-    // Park the chunk and one promise record; the backpressure -> false
-    // flip delivers the cached continuation (see
-    // transformStreamSetBackpressure) at the same microtask position as
-    // the old [[backpressureChangePromise]] reaction. The continuation
-    // resolves the sink promise with the perform-transform promise, so
-    // adoption reproduces the old derived-chain settle depth exactly.
-    // The writable dispatches a single write at a time, so one pending
-    // slot suffices.
-    assert(state.pendingWrite === undefined);
-    const pendingWrite = PromiseWithResolvers();
-    state.pendingWrite = pendingWrite;
-    state.pendingWriteChunk = chunk;
-    state.writeContinuation ??= () => {
-      const pending = state.pendingWrite;
-      const pendingChunk = state.pendingWriteChunk;
-      state.pendingWrite = undefined;
-      state.pendingWriteChunk = undefined;
-      const writableState = state.writable[kState];
-      if (writableState.state === 'erroring') {
-        pending.reject(writableState.storedError);
-        return;
-      }
-      assert(writableState.state === 'writable');
-      pending.resolve(
-        transformStreamDefaultControllerPerformTransform(
+  if (stream[kState].backpressure) {
+    const backpressureChange = transformStreamBackpressureChangePromise(stream);
+    return PromisePrototypeThen(
+      backpressureChange,
+      () => {
+        const {
+          writable,
+        } = stream[kState];
+        if (writable[kState].state === 'erroring')
+          throw writable[kState].storedError;
+        assert(writable[kState].state === 'writable');
+        return transformStreamDefaultControllerPerformTransform(
           controller,
-          pendingChunk));
-    };
-    return pendingWrite.promise;
+          chunk);
+      });
   }
   return transformStreamDefaultControllerPerformTransform(controller, chunk);
 }
@@ -715,15 +661,9 @@ function transformStreamDefaultSinkCloseAlgorithm(stream) {
 }
 
 function transformStreamDefaultSourcePullAlgorithm(stream) {
-  const state = stream[kState];
-  assert(state.backpressure);
+  assert(stream[kState].backpressure);
   transformStreamSetBackpressure(stream, false);
-  // Park the pull: the next backpressure -> true flip delivers the
-  // pull-fulfilled step (see transformStreamSetBackpressure). The old
-  // [[backpressureChangePromise]] this replaces was only ever resolved,
-  // so the parked pull needs no rejection delivery.
-  state.pullPending = true;
-  return kParkedAlgorithmResult;
+  return transformStreamBackpressureChangePromise(stream);
 }
 
 function transformStreamDefaultSourceCancelAlgorithm(stream, reason) {