I don't know about making it slower, but the hotter is due to build up without the ability to vent as easily. I've noticed some cases trap more warmth in, but not to the point where it would hurt performance.
I'm going to try one of those shield skins tonight just for the heck of it. If it goes on without any issue I'll keep it, but I'm skeptical of the entire install process(I'm not spraying anything on my phone. I'll steam the bathroom shower up and reduce dust that way.