I have the same problem, powerful computer, but I always get "buffering" while previewing (the more clips, the worse). Changing the preview resolution or the fps doesn't help. The preview also gets stuck nearly after every clip/scene. And once the preview is running, I often can't stop it anymore - the video is stuck, sound is still playing, but videopad does not respond to any interactions. Sometimes I have to wait until the whole sequence is over, before I can continue working with videopad.
However, I think I know the reason for that. If I change the preview fps to any value, the preview doesn't get stuck after one clip (no buffering) - unfortunately for a few seconds only. I'm pretty sure, it has something to do with the background queues (videopad is always working in the background). These background processes obviously are so demanding, that the preview doesn't have enough CPU time to work correctly. Unfortunately there is no way to stop these background processes...