There is a nasty bug in the current build related to terrain tiles loading. Some cards are more affected than others (I've got some serious stutters for example). If the framerate (ctrl+F12) is stable while you are not moving but drops as soon as you move the camera, it's that thing.
Reducing the terrain resolution helps a bit, but you'll still have serious drops (and tiles not visible around some mountains).
Apparently, the problem was identified so expect a fix in the next build (hopefully).