Disable mpbios and acpimadt.
rebooted (which took also a while), at the boot> prompt did boot -c to enter the kernel config, then at the UKC> prompt did: disable mpbios disable acpimadt quit boot speed was then... normal, the VM performed normally, and the kvm process didnt consume cpu resources from the host. logged in as root, edited /etc/bsd.re-config to contain disable mpbios disable acpimadt so that it was applied on upgrades on the newer kernel linked at the end of the upgrade process.
Source: Landry Breuil at 2024-02-21 15:42:02 CET on Proxmox BugZilla - Bug 4097 - 32-bit OpenBSD is unbearably slow (50x slower) after upgrade to Virtual Environment 7.2-4
See also: Narkive - misc.openbsd - Very slow I/O under OpenBSD i386 on qemu-kvm from RHEL7rc