Have you felt the motor after it slows down? If the motor is fairly hot or smells like it is hot you may have found your issue. Some of my machines have motors that get relatively hot and tend to slow a bit on prolonged sewing, but friction with lint and built up crap in the works can really cause a motor to work too hard and cause it to slow. After the motor cools the speed comes right back up but then slows again as the overall friction starts to mount again.
There actually is an oil hole on the majority of sewing machine motors, one on each end right near the shaft entrance and the other near the end cap. A single drop of oil in that hole annually (depending on your usage) can stave off motor bearing failure for a long time.
The first thing I do when I acquire another machine is run the motor flat out for a bit to see how many sparks the carbons throw and how long it takes to heat up. That usually gives me a pretty good idea of how the machine was used in the past. It also allows you to watch the motor burst into flames if it is clogged up with lint or oil or stuff. (My wife seems to object to me doing that in the house....)
As mentioned, high belt tension can do the same thing and not allow the machine to reach full speed but is sounds like the belt has not been messed with, so I am up for high load leading to heat build up and slow down.