When we transitioned all services but mediawiki to codfw, we saw the eventgate latencies spike up for MediaWiki, which was kinda-expected given we were going cross-dc:
These are the eventgate latencies seen from servers in eqiad.
Now we expected latencies to go down once we transitioned mediawiki to codfw. Instead not just it didn't go down, but we also saw a very high latency in restbase (which was transitioned together with eventgate to codfw)
This seems like a possible cause of T261763 and is a very serious performance regression.