Proposal: Add "Use a DOSBox config with core=simple, cputype=386_slow, cycles=3000." to the rules for DOS categories.
Ultima 6 for DOS has the same problem that many old DOS games have: The game runs to the speed of your hardware, so the game can be made to run very quickly just by throwing it on a fast computer at a high frame rate.
The Jazz Jackrabbit community (http://www.speedrun.com/jj1) tries to address this by standardizing a DOSBox config to use for valid runs. The same strategy should work here. Out of laziness or simplicity, I've been using the config that ships with the GOG edition of the game (core=simple, cputype=386_slow, cycles=3000), which seems as good a standard as any.
Standards are really good to set so that games all run at the same speed. I'm completely fine with this and will update the rules for it.