I just updated to 6.24 and was having a slew of weird problems. I think maybe the update reset a bunch of settings. I had the audio export glitching with that codec that you have to delete and have the program redownload for you, and up until just right now I was suffering through impossible preview lag and buffering time.
I right clicked the preview window and set the preview down to 360p (it was at 480p when I looked) and it's working great again now.
If your computer is strong enough, you shouldn't need to create low-res copies and edit with those and then "change file" back to the hi-res ones. I mean it's a solid suggestion but it might be circumvented by some tiny change like this instead. My ancient laptop was working with multiple gigs of videos up until I got my new PC a few months ago. I think for the most part, most computers should be able to find a settings combination that work for it.
Anyway Videopad is not without its fair share of infrequent irritations, but it's powerful and versatile and I have stuck with it this long so hopefully you can find the quick fix and not have to get your money back. Good luck!