I’ve seen this problem outside of Webflow, too, and I think it’s an affect of 1) the video having fluid dimensions 2) the browser having to process the video + white opacity.
Here are a few ideas to try. I hope these help:
Give the video a fixed width and height and center it inside a variable width/height frame
Figure out an alternative to fade-to-white
Keep the fade-to-white action, but detach it from scroll, so the browser has to work a little less to make it happen