Hi @videopiet, 
Welcome to the Premiere Pro forums. Thank you for reporting a problem and for providing a video showing the issue. Just confirming you have the GeForce RTX 3060 and using the latest verison of the Studio driver? 
 
It's a little hard to tell from the video but what file types are you using (is it a mix of MTS and MP4)? And where are the files and project located? 
Noticed there is a process happening in the Progress Dashboard up in the top-right corner—what process is occuring and 
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