I have a somewhat older computer, and it's fine with most things, but on super high-res video it's choppy and the CPU's at 100%. Should I downsample these videos, or is there a setting that I can use to make them play without stopping?
You have to downsample them before playing them. (Might take a few hours and you loose quality.)
The hard work is really decoding the video and not displaying it. That means no matter if you display a lower resolution video, the work is all the same.