At mega-graph speeds (329 sps), the diag sync_and_swap blocks for
100ms+ because the DtoD copies haven't finished in the 3ms step time.
Gate sync+snapshot to every 10th step (or log/checkpoint boundaries).
The DiagFrame still sends every step using stale staging data — the
background writer drops most frames anyway via try_send(1).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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