I have a similar engine, the 785. My father has the 1-700E, also similar. Neither is a speed demon, but both are reasonable pullers, and that without traction tires or magnetraction.
Expectations about how much a loco should be able to pull are way higher today than 30 years ago, when that engine was made. But you don't say that it won't pull the train, just that it is slow. An engine without traction tires is more likely to slip than stall when pulling too many cars, so as long as it is pulling the train at all, it is probably working in the speed range it was designed for.
Is something still wrong, even after you tuned it? The unscientific way I would go about finding out would be to run it for a solid hour. After it gets good and warmed up, check the voltage it is drawing. If it is still running at about the same speed at that same voltage an hour later, then you have a healthy engine. If gets hot and starts going slower and slower, then there is a problem.
