I have tried kernel 5.4-rc5 =E2=80=93 that did not help.

Disabling cpufreq boost (echo 0 > /sys/devices/system/cpu/cpufreq/boost)=
 stops
the drastic slowdown from happening, but this also makes the system
significantly slower than normal (still much better than after the bug
appears), so there is no gain from using the discrete GPU (DRI_PRIME=3D1).

I wonder if this might be the problem:

> $ cat /sys/class/hwmon/hwmon3/{name,temp1_crit,t=
emp1_crit_hyst}
> amdgpu
> 94000
> -273150
> $ cat /sys/class/hwmon/hwmon4/{name,temp1_crit,temp1_crit_hyst}
> amdgpu
> 80000
> 0

Are these hysteresis values being used? If so, then those values won't work=
 for
recovering from thermal throttle. Though, I have never seen temperature rea=
ding
reaching 80 degrees there.

The first one seems to be the discrete GPU (RX560X), the other one integrat=
ed
(Vega 8).