You can increase the CPU-cycles in DosBox with CTRL+F12 and decrease with CTRL+F11, but don't set them higher than your CPU can handle, else it will be even slower again.
The only other way to play Master of Magic is to run it in the Dos-prompt in Windows 98, but then it's little too fast there, so you'll need Slowdown for Windows:
http://newton.pbk-solutions.de/ncu_nbu/slowdown.htm
It won't even run under Windows XP by the sound turned off, I think...
