"does fast or slow pan/scan render better (meaning once compression is applied)"

Slow is better, well I guess if it's fast enough, like in the blink of an eye fast, no one will notice but otherwise with any temporal compression the less changes between frames the better.

That said, screen capture is usually pretty clean and with large areas of identical pixels, so it's pretty easy on the bandwidth. Still, if you're using low bandwidths stick to slow.

Bob.