Quite a lot, actually. Mark posted a demo of Markio with a cubemapped sphere in the castle. That was done realtime, and it shows the speed hit you can expect. I think it came with the 1.85 update, but if not, I'll see if I can find it.
The speedhit comes from having to CopyRect the rendered images to the texture. If it were possible to render to a texture, it would be much faster. Cubemapping itself is very fast, if you use a static texture. But a dynamic texture causes a fair amount of overhead, particularly for low end videocards, and you might want to consider only updating 1 face each frame or only updating every 5 frames or something like that.
The speedhit comes from having to CopyRect the rendered images to the texture. If it were possible to render to a texture, it would be much faster. Cubemapping itself is very fast, if you use a static texture. But a dynamic texture causes a fair amount of overhead, particularly for low end videocards, and you might want to consider only updating 1 face each frame or only updating every 5 frames or something like that.