For those who don't remember, the turbo button existed as a backwards compatibility hack. A lot of games in the early days did dumb stuff like implement game timing loops by spinning the CPU, because devs just didn't think ahead to "what if people play my game on hardware that doesn't exist yet". So when the first big leaps in clock speed came out, some games ran unplayably fast. The fix was a literal button on the front of the machine that controlled the clock speed.

Devs learned how to use the timer chips pretty fast after that though, so for a long time the turbo button existed but appeared to be useless. Pushing it would toggle a light but nothing appeared to change.

And it was inconsistent between vendors whether light on meant fast or light on meant slow.