The purpose of the Turbo button on PCs was indeed to slow it down, originally to the 4.77 MHz of the original 8088, for compatibility. For marketing reasons it

The purpose of the Turbo button on PCs was indeed to slow it down, originally to the 4.77 MHz of the original 8088, for compatibility. For marketing reasons it was designated “turbo” instead of something indicating a slow-down. Turbo mode was always the default, since that was the standard speed of the respective CPU.