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.)