The game updates internal state at a rate of 20 turns per second. Drawing it at a higher Frames/Second than that will just waste GPU time, since you'll be redrawing the same frame multiple times. The game won't look any better or play any faster.
 Mostly. If it falls behind, such as due to CD spin-up latency, it will run fast to make up the missed turns, then return to a 20 turns/second rate when it has caught up. This killed more than a few players who set off a long CD spin just as their character awakened a dangerous enemy, then when the game resumed, it played 10-15 seconds of game time in less than a second of real time, allowing the enemy to overwhelm the player character before the human could react. The Butcher and Lazarus were the most common problems, since both have special activation voiceovers.