Just for a quick check, measure across the wide face of the V-Belt. It should not be less than 1/2". If it is, the belt is worn, it can drop too deep into the sheave set and begin binding just a bit.
It can bind so much that it freezes the motor entirely, or sieze up under load and more bad things can happen.
It may be a contributing factor. If you go to the main page of My Google Drive and review the document that started the entire thread viewtopic.php?p=185690#p185690
a few years ago titled "Buying a Used Shopsmith V2.PDF" you will find a series of photos with hot links back to threads here, that displays how it should look.
Equally, the document Bill Mayo - Mark V Headstock Improvement - Repair Kit for Speed Control and Eccentric Bushing v1.01 13 Jun 2016.pdf that can be found in the Adjustment and Alignment Folder can be really helpful getting the speed dial set up correct, especially important when replacing the V-Belt.
Like others, I would love to see the pictures of what you have.
The motor slowdown can be tied to your internal wiring in the headstock, the cord that is powering the headstock, an extension cord, or even a loaded circuit, the condition of the blade sharpness or surface condition if cutting... a host of things outside of the motor itself.
Can you elaborate some on the conditions of those parts?