I found a small amount of info over here that may help. They recommend cutting the MDF at 20-25 cuts per inch, and give a formula to work out the feed speed in ft/min thus:
(RPM x no. of cutting edges) / (100 x cuts / inch)
So if you are cutting at 20K rpm, using 2 flutes, your feed speed should be 192 IPM.
So you could be right that your router is slowing down at low RPM's, since lower RPM and higher feed would be the way to go.
Sorry I can't really help much, I don't do that much MDF work