Maybe this is asking too much, but do you have a timer somewhere in your household? A watch with a built-in system? You could just set this to "whatever your goal is", or really anything that's above the time you want, and look at it and compare to when you started. If you're aiming for Speedrunner Gold, for instance, set the timer to 20 minutes and start it when the music begins to play on Base Entry. It shouldn't be hard to look slightly away from your screen in between levels.
Again, though, for something like this, even if it WERE to be added, definitely make it a config toggle. Playing a turn-based game, I can't imagine a lot of people wanting to see a clock ticking away while they play.