When traversing chunk boundaries and chunks at the edge of the render distance are loaded and being rendered, a significant amount of chunk updates are performed per frame (I have observed anywhere from 2/300 up to about 600 depending on render distance)

This has been tested on a variety of render distances from 3 up to 16 and appears to affect all of them.

If the player stays within the boundaries of a single chunk then the chunk updates reduce drastically and FPS returns to normal. It only happens when chunks are loaded into render and require updating.

My guess just by the sheer amount of chunk updates is that it updates all chunks loaded or something like that and it's possibly in aid of fixing the "world doesn't load" issues.

For me I average at about 100-120 fps above ground in a normal world but now my fps drops to around 24 on any render distance while I am moving (specs above for comparison)