Unusable slow. Loaded a guest VM the browser was juddering, CPU high on any process that was trying to do anything (that was when I started wondering on what was going on). Restarted the guest the VM when starting showed a starting bar along the bottom, I have never seen this before, so think it normally clears quickly before being displayed. Then I watched a starting bar on each slow VM. The VM bios text displayed one line a time, then clears one line at a time. The choose a boot device screen displays a line a time. I did not even need to boot into an OS to work out it was slow.
Switched off Enable hardware-assisted virualization and rebooted, like a light switch, day and night difference. Now back to normal, full speed bios displays normally.