Chunks aren't be generated when player is riding on a horse.

When I'm riding on a horse chunks aren't be generated and when i try go to on this chunks game lags (no 0 fps) and all mobs etc. stop walking, this lag is like a lag when on server player didn't have internet.
BUT when i dismount horse before this lag begin, game back to normal and all chunks is be generated without any problem.
(I'm sorry for my bad English)