diff --git a/CHANGELOG.md b/CHANGELOG.md
index a7eb45b..836486f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,20 +12,32 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Add `Tests/StreamingScannerPerformanceTests.swift` (issue #4), measuring
   `scanStep()`'s overhead over `scanAll()` on the largest real-world fixture
   (github-rest-api.yaml, 9.5MB) across a spread of chunk sizes, gated behind
-  `PUREYAML_STREAMING_PERF_MEASURE=1`. Measured, not guessed: small chunk sizes
-  (16-256 scalars) show a real, chunk-size-dependent rescan cost (up to ~6x at
-  16), consistent with the checkpoint-and-retry design re-scanning a fetch
-  attempt once per chunk it spans. But a control run with `isFinal: true` from
-  construction, where `ranDry` can never be set and no rollback ever happens,
-  still showed ~1.5x overhead over `scanAll()`, proving a separate,
-  chunk-size-independent cost dominates at any realistic chunk size (4KB+): the
-  checkpoint was being copied on every fetch even when it could never be used.
-  Fixed by skipping the checkpoint entirely when `reader.isFinal` is true.
-  Re-measured: the isolated control improved from 1.49x to 1.38x, a real but
-  modest gain; the large-chunk-size ratios (4096+) were unchanged within
-  measurement noise (~1.5x). The remaining floor's cause is not identified.
-  Closing it needs a real profiler, not further guessing, so it is left open on
-  issue #4 rather than patched further on speculation.
+  `PUREYAML_STREAMING_PERF_MEASURE=1`. First pass ran in debug mode (release
+  test builds hit an unrelated `PureYAMLJSON` test-target issue) and showed a
+  ~1.5x floor even in a control with `isFinal: true` from construction, where
+  `ranDry` can never be set and no rollback ever happens; fixed by skipping the
+  checkpoint entirely in that case, which only closed part of the gap in debug
+  numbers (1.49x to 1.38x).
+  `sample`(1) profiling of that debug build pointed at `mutating throws`
+  methods on multi-field structs (`consumePlainContent`, called from every
+  plain scalar), which is a known trigger for defensive retain/copy traffic
+  under `-Onone` that the optimizer removes in release. Confirmed by working
+  around the release-test-build issue with `-Xswiftc -enable-testing`: in
+  release, the `isFinal: true` control measured **1.04x**, essentially batch
+  parity, meaning the checkpoint-skip fix was actually a full fix for that
+  case, not the small, unexplained one it looked like in debug mode.
+  What remains, correctly explained rather than guessed at: a chunked run's
+  `isFinal` only becomes true once, on the last chunk, so for any input that
+  arrives incrementally, the checkpoint stays live for virtually the entire
+  scan no matter how large the chunks are. Release-mode chunked ratios: 11.70x
+  at 16-scalar chunks down to 1.75x at 1,000,000-scalar chunks (10 chunks
+  total for this file), never reaching the 1.04x no-chunking floor. Closing
+  that gap needs the deferred persisted mid-token suspension, which removes
+  the need for a checkpoint at all rather than making it cheaper; the
+  checkpoint's cost cannot be optimized away further without it, since
+  `isFinal: true` is the only condition under which skipping it is provably
+  safe (a single scalar can span past any finite "probably buffered enough"
+  margin).
 
 - Broaden the real-seed mutation property suite (issue #6) with three mutation
   classes named in that issue but not yet covered: `invalidControlCharacter`
diff --git a/Tests/StreamingScannerPerformanceTests.swift b/Tests/StreamingScannerPerformanceTests.swift
index 49ab77a..7815e39 100644
--- a/Tests/StreamingScannerPerformanceTests.swift
+++ b/Tests/StreamingScannerPerformanceTests.swift
@@ -12,6 +12,17 @@ import Testing
 /// practice before persisted mid-token suspension (a materially larger, riskier
 /// change to the scalar-scanning loops) is worth building. Gated: release-mode
 /// timing on a 9.5MB fixture is not a default `swift test` concern.
+///
+/// Run this in release mode; debug-mode numbers are not representative (Swift's
+/// `-Onone` inserts defensive retain/copy traffic around `mutating throws`
+/// methods on multi-field structs that the optimizer removes in release). This
+/// package's own test target needs `-enable-testing` explicitly in release,
+/// working around an unrelated `PureYAMLJSON` test-target issue:
+///
+/// ```sh
+/// PUREYAML_STREAMING_PERF_MEASURE=1 swift test -c release -Xswiftc -enable-testing \
+///   --filter StreamingScannerPerformanceTests
+/// ```
 @Suite("Streaming Scanner Performance")
 struct StreamingScannerPerformanceTests {
     @Test("measure scanStep() overhead over scanAll() at a spread of chunk sizes")