Sounds like a "universal" type motor. Worn brushes is my first best guess. An internal short in the armature is evidenced by excessive sparking at the brushes, odor and reduced speed. Testing for internal shorts on the armature requires a "growler" and disassembly of the motor.

Have you removed the brushes to look at them?
When you do, blackened contacts on the armature are a good indication of poor contact. If the contact area is pitted, that's an indication of heavy use with poor contact. Increased sparking is a symptom that the brushes are deteriorating.

"Have you removed the brushes to look at them? "

Make sure you mark them so they go back in exactly the same way on the same side they same from.

Swapping or turning brushes (even 180 degrees) is a great way to destroy armatures.