Have very happy installs of fully-updated 18.04 working on my Mac Pro 1,1 until I try to update to the HWE kernel. This causes terrible performance problems upon reboot, locking one of the CPU cores into 100% usage constantly. Happens every time I install HWE from clean 18.04 installs.

The same behaviour is seen with clean installs of 19.04 and 19.10 too.

Common theme here might be the v5 kernel. Anyone else seeing this on their early Mac Pros?