I don't think my experience is unusual. A lot of people can't get P5 to cancel at all. It was improved with SR4, but still isn't perfect. (However, I will definitely try using ESC instead next time!)

My specs: Pentium 2.8, 1.5 Gb RAM, XP Home. Poser is on a 120 Gb hard drive that's more than half empty, my runtimes are on a 265 Gb hard drive that's about half empty.

The "Room of Mixtures" images in my gallery locked up Poser when I tried to cancel. They do use very high-res textures.

The infamous P5 memory leak is also a problem. I suspect that if I just opened the Poser file, started a render, then cancelled, it would probably cancel. But work on the image a couple of hours, try several test renders, then bail out on the final render because I forgot to uncheck "smooth polygons"...it would likely lock up.