Well, if you have 4 identical models, and the deciders are set the same way on all 4, and one is running differently (slower) than the other 3, then most likely it is a mechanical problem. As for what specifically - that is difficult to asses at this point. Could be motor, lack of lubrication, or mechanical binding. I suppose there is a chance that the decoder is failing and causing the problem, but it seems more likely to be mechanical issue.
For troubleshooting purposes you could swap the decoder between the misbehaving and good decoder and see if the problem follows the decoder. If it does, then the decoder is faulty, if the problem stays with the model, then it is a mechanical problem. If the tender can be disconnected from the loco, that would make the decoder swap easy -- just swap the complete tenders with the decoders inside. Owning multiple identical models makes troubleshooting easier.
