Thanks.
I created a number of terrain maps of Mars this weekend with a grid of ~320 points on a side (= ~101K squares, 203K triangles). Everything seemed to work fine for a while, but after viewing several models for a while, the frame rate would drop. (It was a LiDAR-based device, which I'm guessing added a lot of its own mesh for occlusion purposes.)
I'll do the next batch of terrain models at at a resolution of 224 points on a side to see if that helps.
(Way back when I took computer graphics courses, we did everything in triangles. I guess I should keep that in mind.)