Specs: Ryzen 5 1600 3.6GHz, Prime B-350 Plus, 16GB DDR4 Ram, Radeon RX 580 8GB (Win10 1903 with latest updates)
Radeon Version: 19.9.2 (and 19.5.2)
I'm having the same issue. Videos play back very choppy even at 1080p using vlc, windows media player and movies & TV app. GPU usage spikes to around 100% in task manager (video decode).
I'm currently using Radeon 19.9.2. I upgraded from 19.1.1 which didn't have this problem. This issue has been in previous Radeon Versions too, namely 19.5.2, which caused me to go back to 19.1.1. Now I need the newest driver for support of new games (Gears 5 for example).
I hope this video playback issue will be adressed, as it is really annoying. Like velo_kun said, it can be temporarly fixed by restarting the computer.