ORIGINAL: foxy42
If you can't do it via the transmitter functions, the only way is to reduce the voltage going to the servo, or buy slower servos. Another option is reducing the end points (your controller will surely have this function), this will of course limit your total maximum movement, but it will also result in stick movements having smaller effects. Adjusting the dual rate setting may also be an option (once again, if your tx has the function).
DO NOT reduce the voltage. This will make them have less power and perhaps quit working at all depending on how much you lower them and isn't that easy in the first place.