Hi Kukobar,

The first thing to try is to turn off viewers you don't need from the Viewers menu.  After that, try changing the render options under View -> Render State to see if you can get some performance back that way.  Another thing to try would be to adjust the clipping planes under View -> Camera -> Camera Settings.  I recommend adjusting those settings first before trying to simplify the display geometry for your heightfield.

cheers,

Josh