diff --git a/notes/observed-bugs.md b/notes/observed-bugs.md
index 7fbe8e7df..8016e5da0 100644
--- a/notes/observed-bugs.md
+++ b/notes/observed-bugs.md
@@ -82,25 +82,47 @@ without board power-cycle").
 **Status**: task c3 (indirectly, via bes_chardev removal which currently
 gates the signal/nosignal mode switch path).
 
-## Backlog — full architect review of bes2600 driver code quality
+## Architect review — now BUG-#5-blocking (was backlog)
 
-The Phase 0 perf trace for Bug #5 exposes a "when in doubt, add a lock"
-pattern in the BH path (~20 % CPU in `_raw_spin_unlock_irqrestore` even
-during healthy throughput). Markus has flagged this for a separate
-architect-review pass: have Claude Sonnet (or equivalent reviewer) do a
-top-to-bottom code-quality review of the bes2600 sources we have on
-boltzmann (`~/src/besser/bes2600-dkms-mobian/bes2600/`), looking for:
+The Phase 0 perf trace for Bug #5 first exposed a "when in doubt, add a
+lock" pattern (~20 % CPU in `_raw_spin_unlock_irqrestore`). The
+follow-up ftrace measurement (2026-05-07 17:00) refined the root cause
+to an architectural problem: **the bes2600 driver dispatches every
+SDIO transaction through the kernel workqueue**. Numbers from a 3-min
+4 MB/s ohm capture (post-reboot, srcversion `1B3B3ED0`):
 
-- needless lock proliferation
-- BH / workqueue dispatch shape
-- error-handling coverage
-- dead code / leftover-from-cw1200 cruft
+```
+wsm_cmd_send:               13/sec    (host-to-chip command rate, surprisingly low)
+bes2600_rx_cb:             611/sec
+bes2600_bh_wakeup:         267/sec
+lock contention_begin:      50/sec
+workqueue_execute_start: 5,643/sec    ← DOMINATES; matches the mmc
+                                       transaction rate from earlier perf
+```
+
+5.6 k workqueue dispatches per second is the throughput floor — not a
+specific lock, not WSM-command rate, not decrypt-state. A surgical fix
+to any single function won't move the floor; the architecture needs
+to be restructured to amortise SDIO transactions across fewer work-
+items (or move SDIO RX out of the workqueue entirely).
+
+This is where the **Claude Sonnet architect review** belongs: a
+top-to-bottom assessment of `~/src/besser/bes2600-dkms-mobian/bes2600/`
+focused on:
+
+- the workqueue dispatch shape (most actionable)
+- needless lock proliferation (the original signal)
+- BH / RX scheduling boundaries
+- error-handling coverage and dead-code from the cw1200 ancestor
 - API contract violations relative to mainline mac80211
 
-Output: ranked list of cleanup targets that would make later patch series
-land more cleanly. Not blocking on Bug #5 — independent track.
+Output: ranked list of restructuring targets, with predicted-delta
+estimates against the Phase 1 metric (≥ 2 MB/s sustained @ 4 MB/s cap,
+< 10 % CPU in lock-cycling, no link cascade in 30 min).
 
-**Status**: backlog. Schedule when Bug #5's measurement pass finishes.
+**Status**: now blocking on Bug #5 (was independent track). Surgical
+patches B5-1, B5-2, B5-3 from the original Phase 4 candidate list are
+all DEFERRED until the architect review's restructuring map is in.
 
 ## Bug #5 — RX path degrades under attempted-throughput pressure