There have been times when I forgot to resize a map, and the program basically ground to a halt. I use large maps that are 100 pixels to a 5' square, which work just fine, but when you load up a hi-res image (250 pixels per square high-def png, for example,) you're gonna have a bad time. As I understand it, the program only uses one processor core in its current iteration, but the version they're building on the Unity platform will be multi-thread, multi-processor capable.
Looking forward to that.