When trying to green screen a video thats 1 minute long, the video ends up being super glitchy

Is anyone elses tiktok like this? When I upload a 15 sec green screen the video plays normally but when I upload a 60 second video that I want to use as the green screen then the video is all glitchy and doesnt play smoothly.

Edit: it only seems to happen on videos which I first put together in capcut and then try to greenscreen. If I use a video thats 1 continuous clip and doesnt cut through scenes then it doesnt glitch.