21w15a caves datapack freezing 21w16a engine

Loaded the latest snapshot. Downloaded the Caves datapack from minecraft.net. Created a fresh new local world. Began to play single player - basic setup, crafting table, furnace, dug about 300 blocks - and the game started freezing.

You walk a bit, the screen freezes in place - no mount movement, no button press response, a second or so later, all movement is restored, you start walking, a few seconds after that you're locked up again.

This has not occurred on my system on any other version.

What I expected to happen was...:
Smooth gameplay.

What actually happened was...:
As described

Steps to Reproduce:
1. Play the game, the freezing will present within 2 minutes.