xhyve supports FreeBSD now! Converted my VirtualBox machine to a raw disk image using VBoxManage clonehd, started it in xhyve. (Had to Esc to the prompt, unload and disable-module vesa to prevent the “module vesa not found” error.)

Works great! Rebuilt the whole system (clean buildworld installworld buildkernel installkernel) in 32.783 minutes. VNC is a bit slower than VirtualBox’s display emulation, but that’s fine.