In theory TDs have good speed until you take into account terrain. Then TDs become liability.
Wrong for the topic at hand.
We are talking about Barbarossa and then there is really only two terrains you need to worry about except plains, forest and woods.
TDs have -30% movement for forest and -10% for woods.
MOT have -30% movement for forest and -10% for woods.
ARM have -40% movement for forest and -10% for woods.
MEC have -30% movement for forest and -10% for woods.
Starting to see a pattern? Yes TDs are equal or better then all of the above they can be combined with, for the relevant terrains in Soviet. I couldn't find any other terrains where they have worse stats then Mechanized either except for 10% in mountains.
So if you get them to the same speed as MEC in the ledger, then that is what they will perform on the field too, except a slowdown of about 1kph in mountains which I think I can live with.
Your fieldtests are not really relevant, if you research armour but no engine for Tank destroyers then ofcourse they can become slow as hell. Or they could have lagged behind by 1 hour in the mountains which means the other divisions take all fuel and leave them at 10% of max speed until new fuel can arrive.
AC are the best for speed where units are super fast. Eng are the best there units speed is just above 8 or lower.
So why did you suggest engineers for Mechanized then? :S