[Rafael Amador] “Really. Gif files have only 8b per pixel. Only 256 possible colors in a picture.”
It’s the same reason people use JPEG, not GIF, for photos when building webpages.
In general, any additional compression by a lossy codec can amplify the noise. Of course one can smooth it to clean it up. In either case that absolutely NOT the way to analyze noise issue. One needs to look at the original files. One needs to examine on a monitor with high enough resolution to see that detail.
You can’t even judge these things accurately in an NLE interface since it may be doing some fast scalling. That’s why people use output to a monitor while editing.