Interestingly I am getting about equal 1080p playback performance with local videos on my X5000/020 under AmigaOS 4.1 FE with Emotion video player and the composite comp_yuv2 output with a Radeon R9-270X and under Linux using a Radeon FireMV 2260 pciex1 in a pciex1 slot with hardware assisted video playback. I am able to playback 1080p avi and mp4 videos with frame skip enabled . 1080p mkv's are a problem for both OS'es .
720P playback is better with the same setup under Linux. Youtube videos can be streamed ok.