I would not have expected it to hurt at all on a Cub, in fact most airplanes would not suffer from taking a bit of speed out of a servo. Back in the day most standard servos would have a speed rating of .18 to .20 and we were able to fly just fine. I get where there are cases where a faster servo is desirable but for most guys if slowing a servo down will make an improvement with the particular airplane they are flying then I say go for it.