Lag spikes when going to a new chunk on the positive X or Z axis.

When moving from one chunk to another, there will often be a lagspike if you're going the positive X or positive Z direction (east or south). Negative X and negative Z (west and north) don't cause lag for some reason. This doesn't happen for every new chunk, but it happens more often than not. Only tested in an optimized world (via the Edit World menu on the Singleplayer list), might be different if the world hasn't been optimized.