I guess you would call it chunks glitching. It's absolutely unplayable when it happens. It's just weird. Chunks are duplicated. Things appear which are not really there. It seems you can interact with the world, but I can't see anything at all because of all the glitching. I've noticed that during the time when this is happening, two songs are playing at once - even from the menu screen (which is not glitched in any other way). It happens with worlds created before 1.9 and new worlds. No worlds will open right when it is glitching. Exiting Minecraft completely and reopening will fix the issue temporarily (sometimes), and even then - it takes several restarts a lot of times before the glitching stops. I've also noticed what looks like gigantic mobs moving around. It's all very strange. And frustrating.

EDIT : I forgot to add that I've lowered the render distance to 2 chunks (the lowest setting), and the issue persists.