[Spark910]

Define lag?
Your character slows down, because it is going up or down in depth (height [Z] ) so it slows and speeds up.

Also because you have alot of object NPCs on the gmap, as your terrain is normally the tileset, and you need images and more tilesets for insides and to do inside levels.

So is is actually lag, or just slow down and speed up of the player?