Blizzard did a similar thing in World of Warcraft during the beta. After playing for a while, your character would get "exhausted" and start earning half experience for killing mobs. The only way to stop being exhausted would be to log off or spend a LONG time in an inn. At some point, they flipped the script. They made the "exhausted" state the default, and while offline or in an inn, you would gain a "rested" experience buffer, where you would earn double experience.
The mechanic worked exactly the same, but by giving it different terms, players felt rewarded for stepping away from the game occasionally, rather than punished for playing too long. They also marketed it as a way of giving players a way to "catch up" after spending a day or two offline.
For some reason this feature persisted in PC compatibles long past having any useful purpose, e.g. toggling a 386 between 33 MHz and 25 MHz. Perhaps manufacturers feared any PC without such a button would be perceived as slower, even though as you say, it's really a slow-down button not a turbo button.
Yes of course you'll keep it on the fast speed as much as you can, not the slow speed. But it's still presented as fast being a bonus rather than slow being a malus.