5 hours later... same problem, #Fedora41 and Kernel 6.12 broke #VirtualBox. Same solution, https://forums.virtualbox.org/viewtopic.php?t=112580 of telling KVM to not hog the CPU's virtualisation hardware (or use modprobe -r kvm_intel).

However, networking stopped working too. My little Ruby on Rails server VM couldn't see the outside world, though I could SSH into it from the host OS. Turns out it's a change in the WiFi drivers (Intel AX201) that now ignores promiscuous mode. Gah! Have to run an Ethernet cable now.