The bug

Blocks that break when a piston tries to push them like moss blocks, that also have something else that breaks right after the one below breaks, like tall grass, where the piston is facing east/west causes the game to have a huge lag spike. On the north/south direction it causes a lag spike, but it's far far worse on the east/west direction. It's much worse than it was in 1.16.

West:

North:

How to reproduce

1. /fill ~ ~ ~ ~ ~ ~4 minecraft:piston[facing=west]

2. Place moss blocks below the pistons, and below their heads (see 2021-06-23_11.21.50.png)
3. Place blocks above the pistons, and redstone above those blocks
4. Place tall grass on the moss blocks below the fronts of the piston heads
5. Place a redstone block above the redstone above the pistons