It seems I am not able to play 2x 4k h264 videos at 30fps, even when using vaapi (hardware acceleration).Frame rate averages at about 25fps. Tested using different media players/solutions such as mpv and gstreamer so far.
I've attached a video that shows my performance problem. When the first video launches, playback is successful at pretty much consistent 30FPS. Then the second video launches, and average FPS for both videos hovers around 26-27 FPS average. Closing one of the videos brings the average FPS back to 30 again.