Comment by fipar

The way I remember the turbo button it just halved the clock speed when not depressed, but it didn't bring it back to the original IBM PC's speed.

I do remember using it to make older games play less bad, but, as you say, this rarely worked, I think precisely because the speed was never the original one (I don't remember the clock speed of my first PC, but the first one with a turbo button ran at 33 MHz which I'm sure is a lot more than twice the speed of the first one; seems closer to 8x from what a quick search implies).