I`m not sure but I there weren`t any buttons (cancel, revert, ...) on the preferences page before. Now, after set options to max speed (no images, 4color mode, ...) and hit the use button, then restart it is much better in speed. I noticed when a second screen is open (tried YAM and IBrowse) open xui becomes very slow again. Edit: After some more testing it was also slow without another screen open. This would explain the "sometimes". On emulation you might don`t see that. About how to do things different I can`t help but you may ask on amiga-news forum where you got already help or just stop 68k support so you can ignore the speed issue.
Good news is that it didn`t crashed while playing.