The default speed is just way too slow or bugged. I have a massive(!) ship (currently at just under 9,200 mass) and I put a simple sliding platform that slides out to allow docking then it slides in to conceal the ship in the hull of the mothership. The platform is sitting at 56m x 2m x 23m and a mass of 0.2 (because one of those 2 layers is nothing but mass enhancers). Even with a speed module on the rails making it 200%, it would literally take 10 minutes for this platform to move into position.
Is the speed based on the mothership mass?
Edit: Actually, it does look like it bases the speed off the mothership where the rails are mounted... which seems odd.