OpenBSD 4.3 didn't officially support DRM/DRI, an early version of oga@'s work is included... but you have to recompile both the kernel *and* xenocara.
As for the slowness you're seeing, DRI typically accelerates "3D" graphics, 2D graphics are handled by the Xorg driver, (SIS) in this case.
Here is the man page, which, sadly conflicts with a SiS Ethernet chipset:
$ man -M /usr/X11R6/man sis
I don't see anything that may better your situation though, apologies.