You could try using Speed Limiting in the Options menu.

Ctrl+F11 slows down DOSBox emulation speed with each key press (Ctrl+F12 rises it a gain). You can safely try this, as it's not permanent. When you close and start the game again, it'll be back to default speed.

As Demignon said try limiting the dosbox cycles by pressing ctrl+f11 to see what cycles feels right to you speed wise and then once you got the right speed go into the Ultima 8 folder (Default) C:\gog games\Ultima 8 and look for the file DosboxUltima8.conf and open it with notepad, now scroll down till you find a line saying "cycles=" and then change the value to whatever the cycles were at when the game felt right. Now just save and exit and now when you load up the game it should play at that speed.

Alternatively you could just skip the fiddling around with Ctrl+F11 and Ctrl+F12 and just play around with the cycle settings in the conf file, either way this should hopefully be what your looking for.