What ^^David said. You could test his hypothesis about the e-unit fingers by temporarily hot-wiring around the e-unit and running both locos in forward. See if the slowdown still occurs.
I would extend his recommendations to also include the wire springs that hold the brushes in place. If your loco has been pulling a lot of current, the springs might be annealed, or losing their tension when they get hot. Brush springs are inexpensive and easy to replace. I always solder the "back" of the spring to the brass flange on the brush holder, that eliminates one pressure contact. Many newer Lionels have adopted shunted brushes with a copper braid coming out of them. This conducts current more reliably than the spring touching the back of the brush. I'm not sure whether the shunted brushes have the same resistance as Postwar brushes, or how they might change the loco's performance. If you get desperate and want to experiment, they're cheap and fairly easy to swap in and out.
Finally, perhaps it goes without saying: make sure the commutator is scrupulously clean, including the slots between segments. I've read that an accumulated mixture of carbon dust and oil UNDER the commutator can become conductive when it gets hot- and there goes your performance! Unfortunately on a 2055, changing the whole armature (or removing it to replace the commutator) means pulling wheels :-(
Good luck with your troubleshooting. And please post back if you find the cause!
