That used to be the case when you did a lot of terrain manipulation.

The devs changed that a WHILE back. Terrain manipulation no longer creates lag. Way back, each terrain edit you did would stack infinitely instead of overwriting. This would add up very quick and cause insane lag on large flattened areas or places where you made paths.

The system now overwrites each old edit instead of stacking them. No lag whatsoever from terrain now.