If it's not possible to do it on your radio, or if it's too complicated to do, then you can buy proper devices that plug between the receiver and servo that slow the servo down to a preset speed.
Some models even have an adjustable time.
Heres one I found after a quick look, they are 3/4 down the page:
http://www.galaxymodels.co.uk/new/re...%20Accessories.