Optimizing internal operations of mining pools, it’s likely where the bottleneck is now. See this comment on my stale rate modeling thread that shares a blog post which goes into the performance improvement of using Stratumv2 for pools. The fact that some pools appear to be making their own (!!) blocks stale appears to corroborate this (see here for Antpool mining two blocks at the same height, and here for ViaBTC mining two blocks at the same height).
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