Ok! Solved it, I think. I played around with the Hardware Decoder. After you asked me to make a screenshot, I noticed the Hardware Decoder, I thought I was using DXVA all the time, but Intel Quick Sync was active.
Changed it from Intel Quick Sync to DXVA2 (Native) and after a retry it has nice 25fps without framedrops.
Though, a bit odd it has this behaviour with Intel Quick Sync.