diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index 7597f4b..ac06869 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -5,6 +5,7 @@
 * Test coverage: Added tests for `AsyncSeq.bufferByCount` (obsolete alias of `chunkBySize`) and `Seq.ofAsyncSeq`, which previously had no dedicated test coverage. No functional changes.
 * Fixed Fable CI build: `Microsoft.Bcl.AsyncInterfaces` was pinned to a specific version (`10.0.7`) that was older than the version resolved transitively via `System.Threading.Channels`, causing a `NU1605` package downgrade error that made Fable's project cracker fail during `dotnet fable`. The reference now uses `Version="*"` (matching `System.Threading.Channels`) so both resolve consistently. (#334)
 * Tests: Added comprehensive tests for `AsyncSeq.zapp`, `AsyncSeq.zappAsync`, and `AsyncSeq.compareWithAsync`, which previously had no dedicated test coverage.
+* Performance: Optimised `AsyncSeq.distinctUntilChangedWithAsync` (and thus `distinctUntilChangedWith` / `distinctUntilChanged`) to track the previous element with a `hasPrev` flag and a direct `mutable` field instead of wrapping it in a `'T option`. Previously each iteration heap-allocated a new `Some` box; the new implementation eliminates that allocation, matching the pattern already used by `pairwise`.
 * Performance: Replaced the last remaining `ref` cell in `AsyncSeq.mergeAll`'s internal counter with a direct `mutable` local, matching the `ref`-to-`mutable` modernisation already applied elsewhere in the codebase. Eliminates a per-call heap allocation of a `Ref<int>` wrapper; no behavioral change.
 
 ### 4.17.0
diff --git a/src/FSharp.Control.AsyncSeq/AsyncSeq.fs b/src/FSharp.Control.AsyncSeq/AsyncSeq.fs
index 46cc7ff..6a514ce 100644
--- a/src/FSharp.Control.AsyncSeq/AsyncSeq.fs
+++ b/src/FSharp.Control.AsyncSeq/AsyncSeq.fs
@@ -2740,16 +2740,18 @@ module AsyncSeq =
       use ie = source.GetEnumerator()
       let! move = ie.MoveNext()
       let mutable b = move
-      let mutable prev = None
+      // Use a flag + mutable field instead of Option to avoid per-element heap allocation
+      let mutable hasPrev = false
+      let mutable prev = Unchecked.defaultof<'T>
       while b.IsSome do
           let v = b.Value
-          match prev with
-          | None ->
+          if not hasPrev then
               yield v
-          | Some p ->
-              let! changed = f p v
+          else
+              let! changed = f prev v
               if not changed then yield v
-          prev <- Some v
+          hasPrev <- true
+          prev <- v
           let! moven = ie.MoveNext()
           b <- moven }
 
diff --git a/tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs b/tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs
index ffc140e..39c4569 100644
--- a/tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs
+++ b/tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs
@@ -5258,6 +5258,7 @@ let ``Seq.ofAsyncSeq consumes elements lazily`` () =
   let result = AsyncSeq.ofSeq [1;2;3] |> Seq.ofAsyncSeq |> Seq.take 2 |> Seq.toList
   Assert.AreEqual([1;2], result)
 #endif
+
 // ===== zapp / zappAsync =====
 
 [<Test>]