I've made a ticket MC-238830 of the same issue not knowing about this ticket here which should just be merged but the thing is I did far more testing, so perhaps copy pasting the description of that bug and pasting it on here with edits based on the testing here (or at least some sections) would provide a much more in-detail description of this bug. No idea how to do this from my end though.
New ticket MC-238830 , but just for the server lag caused by opening a buried treasure map in a chest in a stronghold or /locate buried_treasure somehow having to take an unreasonably long time to locate the nearest buried treasure, not the mansion issue that just seems to be an optifine bug.
The odd thing is this seems to have been fixed in the 1.16 snapshots but by 1.16.5 it's still just as big of a bug... then by 1.17 and 1.17.1 it seems to be fully fixed again. Then starting at 21w37a it has resurfaced yet again, which indicates that it isn't a really high priority issue that is easily broken from update to update.
Edit: This turns out to already have been reported as MC-238830 but not listed in the comments here. Probably will just be merged with that one.
The way i play the game, this bug seems like it should have far more ubiquity. That reaction time is pretty much pointless when blocking, that you're effectively punished for doing something to the same effect as parrying, is pretty dumb. But fix it on your own time, Mojang! 🙂 🙂 🙂 🙂 🙂 😉
Can confirm in both 21w39a and 21w40a.
Also Megalodab in this video that does code analysis of this issue I assume it is intended. In short, there is an arbitrary 250ms delay after you hold up a shield until it is permitted to block attacks. I don't know how you would add this to the description like how other reports have a Code analysis section but maybe someone else knows how.
In any case I feel like the delay should be removed entirely (which from what I can understand would involve just changing a 5 to a 0) rather than syncing the animation to the delay because it just adds unneeded jank to the shield and makes you play arbitrarily slow with it, not just with pvp, but also with pve fighting multiple skeletons, for example.