It has to be somewhere on the site that generates the file. I'm not familiar with ridewithgps, but bikeroutetoaster has a speed setting. You can even set different speeds for flats and climbs.
I looked all over ridewithgps, and couldn't find it. It's a real pity, because I really like the way how the site is laid out and how it functions. I honestly don't know how I can calculate what is a "good" vertical speed for climbs in relation to my fitness.