Hi Jeremy, thank you so much for continuing to look into this. I had a lightbulb moment and managed to figure it out. It turns out the problem was with those three particular video files that I had initially changed. Those videos that I put in had an mp4 extension but didn’t have the right codecs or something because were taken out of Elan. I think this prevented them from being run in pcibex. After I realized this, I uploaded yet another version of those problem videos which I think had been edited using a different program and so had been exported as an mp4 file that was compatible with PCIBEX. It worked immediately and now I have no preload issues. I have tested it with someone else who was also able to preload quickly. You may also now be working with the correct mp4 videos.
I think I was lucky because someone else who tried to preload videos had mentioned these issues also and fixed it by converting their mp4 to an mp4 using some conversion software and it also resolved their problem. They were also working with videos originally obtained from Elan, so it might be specific to the exports from that program.
I can send more information about the files if you think that would be helpful to you/others working with video files in the future.