Disclaimer:

The world used in the screen recordings comes fresh from a zip and has been duplicated before opening it to have the same environment in both versions. Both the (same) worlds have been run through optimise world and the cache has been erased.

The actual bug report:

When I load some "fresh" chunks, the chunk rendering in the 1.15 snapshots has trouble keeping all chunks in view visible and causes some glittering/chunks disappearing and appearing. This did not happen in the 1.14.x versions.

Frame drops are also more frequent in the snapshots.

I couldn't append the world used during testing due to its size (150MB).