You start seeing larger environments - so if your computer didn't show signs of struggling before, that would be a sensible time for it to start protesting.
There are some things you can do to decrease the lag, such as changing the detail of graphics in the in-game options window. You can set an FPS cap, as well as decrease adaptive distance for the camera. And in the separate PlaneShift Setup utility, you can downsample the textures. Plus, if you aren't using the most recent drivers for your graphics card, updating might show you a slight performance boost as well.