Mosaic causes css animations in edge to stutter

Hello everybody,
we are developing an html app, that uses Microsoft Edge to output the website etc. (because Edge is the only browser supporting HEVC Videos). This app should be shown on two screens with a resolution of 7680x2160. In the app are some animated elements (svg images with animations, css transitions, lotties rendered on a canvas).
The Problem is: As soon as we enable Mosaic, all animated elements aren’t running fluid anymore, without Mosaic (Edge stretched accross both screens) the animations work perfect. The only fluid “elements” are the videos, they don’t have any problem.

Does anybody know, what can cause this problem and maybe how to fix it?

We have
• two identical 4k screens, both connected via DisplayPort
• a Quadro RTX 4000

We also changed some settings with no success.

Thanks for your suggestions!