256MB RAM is a bit short for Gnome on OpenBSD.
Rule of thumg 512M on OpenBSD, 768M on FreeBSD, for relativelivy high loaded sessions.
With 256MB you need a swap label. This one might slow down the system.
Don't know if it is your case (and you aren't patient enough
, but, IMVHO, if you still can use the keyboard, X might have with the display: display driver, display RAM, swapfile.
Before spending too much time, I first would check for which Intel drivers are available. On other forums, there have been problems reported for the i810 but this was a while ago.
Xenocara -current now build both i810 and the new Intel driver and choses at autoconfigure. For releases and older versions, you would have to try them manually. Maybe use the vesa driver.
If it still "hangs": check swapfile, run concky/gkrellm and top to find the culprit, try to keep your memory usage close to available RAM.
Switch to a less memory hungry window manager.