Minecraft Server Lags when moving through chunk borders in the Overworld. The faster you move through them and changing direction affects how strong and how often it lags. Flying diagonally in a circle through a few chunks creates the biggest lag. The lag always happens exactly when crossing a chunk border.
Entities and other players in those chunks will also be affected by the lag so it does not seem to be client side.
We are currently running 21w17a, we also had the issues in 21w16a before upgrading. We are using the CavesAndCliffsPreview datapack. (Plus 3 others [cratablenametags, server-sleep-by-cthire, TreeCaptiator] that should not affect anything, since they were added after the lags began.)
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I don't think it's a duplicate of MC-197616 because we don't get spammed with "Received invalid biome id: -1". Also we did not use any custom biomes unless the CavesAndCliffsPreview creates custom biomes. I can't see anything in the logs, so I also don't think it is something like the disk i/o lag when writing logfiles as described in the comments there.
I can confirm that this is happening with CavesAndCliffsPreview datapack. It does not appear to happen when that datapack is not loaded.
You can see the lag spikes in my screenshot, I was walking back and forth across a chunk border.
I looked at the pie chart that appears when pressing shift + F3 and it appears that the section "scheduledExecutables" grows every time you cross a border. Especially when you walk back and forth a lot it gets to >30% really fast and it takes several minutes to get back down.
Maybe that might help.
Does MC-166005 happen to describe your issue?
Duplicate of MC-197616