Hold on. With 3000+ troops I had speed of around 20 on flat, 10 in forest and 
0,3m on the bridge while with 2220 trops I reached 24 on flat  with 
8,5 on the bridge. And you say its not a bug, but a feature? 
If that is a feature it should be 
FIXED NOW!!! Being stuck on the bridge for 4 weeks aint' no funny, sir.
[EDIT] Ok, maybe not NOW. It can be fixed together with 1/3 rule  
