PNGs don't slow things down unless you layer them. The basic rule is, if you're going to layer PNGs, make sure the PNGs are as small as you can make them. Be especially careful about the size of the background image if it's a PNG. In one of my templtes I was using a full-screen PNG as a background, and that was overlaying a full-screen JPG. My transparent buttons were also PNGs, and they were slow. So I modified the full-screen PNG to a set of very small PNGs that accomplished the same visual effect. As a result, my PNG buttons were only overlapping the JPG background, and that made the button presses very snappy. I didn't have to eliminate the PNG buttons.

One other thing that slows down button presses is audio. Until the audio on a button completes, the button press is not done. It's easy to overlook the fact that the audio event is what's slowing down the button press.

And again, IR learned from a remote with weak batteries can also slow down a button.