Do I recall correctly that those turbo buttons would, counterintuitively, actually down-clock the CPU? For compatibility with software that had hard-coded timings or something?
They originated with "Turbo XT" class machines which ran an 8088 but at 8, 10 or 12 MHz -- faster than a real IBM PC/XT. Turbo on meant a faster machine, and turbo off meant 4.77 MHz -- fully compatible with timing-sensitive PC software.
Later, in the 386/486 whitebox PC era, some machines had the buttons wired wrong and now it's a meme that turbo made the computer go slower, but that was never true for systems built correctly.
The game was intended to be on a 7Mhz stock machine pre-1990. And it was perfectly timed to that speed.
Sidebar: Ultima games are great. Did you see Nox Archaist?