Since the last Windows update I found out that videos lag in the first seconds when played with uwp app. For eample using Microsoft Film & TV or Microsoft Edge. This does not only apply to internet Videos (YouTube) but also to local files. When I reposition the playhead after 2-4 seconds back to beginning everything plays smooth. I tried it on different machines in office with completely different cpu and gpu all showing those lags.
It does not affect playbakc when using media player or Internet Explorer, so it must be something regarding UWP apps and Video plaback.