I don't know the exact cause of this glitch as it seems completely random which chunk borders cause the lag spikes. However, I can confirm that the glitch only occurs while moving along the Z axis and not the X axis. In edition, it would seem that only chunks within the bounds of the world spawn chunks are effected by this glitch. Also, the lag spikes are purely client-side fps drops and have no obvious effects on the server's performance. However, it is not caused by the computer the game is being run on either, anyone else that happens to come on the server experiences the exact same effect on the same chunk borders. It seems also that loading a now world with the exact same seed does not result in the same effect. The glitch does seem to be specific to version 1.15.1 as loading up a 1.14.4 backup file gets rid of the gitch.