diff --git a/docs/compaction-cost.md b/docs/compaction-cost.md
index d0264a7..feb7f1f 100644
--- a/docs/compaction-cost.md
+++ b/docs/compaction-cost.md
@@ -103,3 +103,46 @@ for the loop, not the symptom:
   in-progress, not idle.** Opposite responses.
 - The tombstone set is also a memory cost: 3.69M entries held 472 MB of a
   15.6 GB box, and it only shrinks when the watermark advances.
+
+## 2026-08-17 re-measurement: the bottleneck moved
+
+The fix that this doc's measurements predate (b3ea690, frame reuse for
+blocks that drop nothing — landed the same day this doc was written)
+changed the answer. Re-measured on the same production box during the
+first completed 4h-interval pass:
+
+| quantity | 2026-08-10 (pre frame-reuse) | 2026-08-17 |
+| --- | --- | --- |
+| observed rate | 3.4 segments/min | ~20 segments/min |
+| segments rewritten | 6,584 of 6,584 | 5,008 of 5,252 examined (244 clean) |
+| bytes moved per pass | (not measured) | ~1.7 TB read + 1.34 TB written |
+| rows dropped per pass | — | 2.3M (~0.01% of the archive) |
+| bottleneck | CPU (zstd recompress) | **disk** |
+
+The disk evidence: a single-stream cold read of one 270 MB segment runs
+~105 MB/s while the pass's 4 workers are active, and the pass aggregate
+(~3 TB moved in ~4h15m) is ~200 MB/s sustained — `/data` is a
+network-attached cloud volume, and that is its ceiling under this load.
+zstd no longer dominates: with tombstones scattered thin (hundreds of
+dropped rows across ~1,600 blocks per segment), most blocks reuse their
+compressed frame verbatim.
+
+Consequences:
+
+- **Pass duration (~4.5h+) exceeds the 4h interval**, so the steady
+  compactor refires immediately on completion: compaction is effectively
+  continuous. Upstream's loop behaves identically at this archive size —
+  shared design cost, not a config error. The interval stays at
+  upstream's 4h default (decided 2026-08-16; do not re-litigate).
+- **More rewrite workers would not help this box**: the volume is
+  saturated at 4. (`--compaction-rewrite-workers=4` vs upstream's
+  default 8 is now a non-difference here.)
+- The O(archive) amplification analysis above still holds exactly — the
+  fold is O(new data), the rewrite is O(archive BYTES THROUGH THE DISK),
+  and frame reuse cut the CPU multiplier, not the byte volume. The
+  sublinear path remains a skip test precise enough that untouched
+  segments are never read — see the FP-union caveat before assuming
+  blooms get there (a candidate set of ~800K record keys unions bloom
+  false positives: at fp=1e-6, expected ~0.8 false hits per segment —
+  every segment matches again; exact per-segment indexes or a
+  threshold-triggered lazy rewrite are the honest design space).