L3DTVi2 does not check what your hardware is capable of rendering, so with L3DT's large maps it is very easy to overwhelm your system. This generally results in very slow rendering (<1fps), and an unresponsive user interface. The following tips may help prevent this:
If the frame rate of L3DTVi2 is really slow, you might want to decrease the view distance using the '7' key.
If the map has a large texture (eg 4096 × 4096 or larger), you may also want to split the map into a mosaic (in L3DT) so that L3DTVi2 can effectively cache the unused parts of the texture out GPU memory.
Lastly, you can wring a few more FPS out of L3DTVi2 by decreasing the ROAM tessellation accuracy ('9' key), and the 'brute force' distance ('-' key).