Running Java 21w17a with the datapack for deep mining on Windows 10, performance is terrible. Movement spends much time in a locked-up state. I see 2-5 seconds of lockout every few seconds of movement. It's much worse flying at higher altitudes, probably due to rendering more distant objects, but even underground in a 1x2 tunnel, there are delays.

But, the worst part is that some game elements are still running in the background. Leaves from chopped trees sometimes decay, so saplings and sticks have dropped during the lockout period, and just are "there" when the lockout ends. I don't know if mobs can spawn, since they don't spawn close to players. But, I did have a creeper move to the bottom of a 2x2 jungle tree I was chopping. It didn't move in but rather was just "there", implying that it moved in during one of my lockout instances.

Without the datapack, performance is back to typical for the game. But, turn on the datapack and the game becomes essentially unplayable. Therefore, I believe this should be a top priority to fix if you want decent player feedback about the second half of Caves & Cliffs.