It's not a speed adjustment, it's a sensitivity adjustment. A subtle, but significant, difference. Some people prefer fewer "wipes," even though it means more rain on the windshield between wipes, while other prefer a cleaner windshield, even if it means more frequent wipes.

A speed adjustment would simply alter the interval between wipes on a time interval basis, independent of how much rain is falling. A sensitivity adjustment essentially alters the amount of rain between wipes, and therefore should speed up and slow down as it rains more and less.