It varied but the older ones definitely had a visible impact if you had something CPU bound running. There was an old helicopter-themed game my dad had which would start running impossibly fast because everything was based on cycle counts rather than actual time.

For most of us it was a misnomer because it was a lag button not a turbo button. It defaulted to on. If you turned it off it would make the machine slower, allowing certain programs to run properly instead of too fast for humans.

But we all have stories of a friend whose machine was slow because they hit the button not understanding what it does. For them it did become the turbo button.

It slowed down the clock to 4.77 MHz on my 486DX2-66.

My 286 went from 8 MHz to 12 MHz when I hit that turbo button. And yeah that was instantly noticeable.

Kept it on except for one ninja game which was unplayable on turbo, using fixed cycle count as delay mechanism. So all enemies and movements were crazy fast on turbo.