I ran into another severe slow down, again while trying to watch something on Youtube. I did find in another thread (here) that running the command sudo cat /sys/kernel/debug/dri/1/amdgpu_gpu_recover will at least get things working again without rebooting, although it resets your session so be sure to save everything before running it. Just wanted to let you know in case you start to experience the slow down issue in addition to the black screen.