The Coinspect security audit raised the following issue with respect to Blossom’s target 75 seconds between blocks (as specified by ZIP 208):
The time between blocks must not be too close to the worst case of block verification time.
The proposed block target spacing is 75 seconds and we believe it could be too close to the
worst case verification time.
I’m trying to find any publicly available discussions of this issue.
This is probably a good start [ZIP 208] [Blossom NU] Shorter Block Target Spacing · Issue #3690 · zcash/zcash · GitHub
The benchmarks in that issue (starting here) show that the worst of the worst-case benchmarks is around 55% of the target 75 second block time on our benchmark server, and even on old hardware it is up to around 80%. Given that these are benchmarking single-threaded performance, there is sufficient scope for performance improvements to zcashd that we are comfortable with the margins. This is also partly why we decided not to go below 75 seconds (because we were less confident that we could optimize to the necessary threshold, while also meeting the other requirements on the ratio between pre-Blossom and post-Blossom block times).