Crazy kernel hacker Arthur Grabowski has redesigned new TLB shootdown code and his work is now in the OpenBSD CVS tree.
That part of the commit message is probably why you want to upgrade your kernel if you’re already running -current on an SMP host:
“This gives a 15% reduction in system time on my dual-core laptop during a kernel compile and an 18% reduction in real time on a quad machine doing bulk ports build.”