Smoke isn't good, but I think from that you can deduce your problem.
There is three kinds of smoke, and three kinds of smoke smells...figuring out that can tell you what your problem is.
If the smoke has a real rubber-burning smell, like an old electric car slot racing smell, then its electrical wires burning. You smoked a motor or at least got the wires very, very hot. This is not likely as it most of the time an electric motor either works, or doesn't. Sometimes you can jump start them by tugging on a pulley, but I have yet to see one run half speed because of an electrical issue.
The burned smell is different. The smoke is whitish in color and far more faint. Maybe just a wisp of smell in the air...not much. That is a bearing that is seized up. What you are smelling is the grease and oil inside the bearing heating up and burning off. This would definitely give you the slow speed you are seeing as the bad bearing is robbing your machine of its power. Still if everything is rotating easy. That isn't your problem obviously.
The third smell is wood burning and it smells very accute. Just a hint of burning and you should smell it very quickly and very easily. That would be your cutting edged getting so dull that it burns the wood. I kind of suspect maybe this is your problem. You said you were planing root balls which inheriently are loaded with dirt and minerals...they should that is how it gets the nutrients from the soil and into the trunk of the tree and up to the crown. I am thinking your knives are dull and are having a hard time planing the multi-directional grain from the root ball. The smoke may also be coming from the belt that is trying to push your wood across those dull knives. Incidentally, even if you have brand new knives on your planer, it would not take long to dull them on root balls in my opinion. In a free spin situation, like where you opened the cabinet and checked, you would see everything as spinning fine, but under a load, it just can't push the log. Well maybe anyway.
Now the disclaimer: I have no idea what you are planing, have for root balls or what shape your planer is in, I am just trying to give you some information to help you out. That is one way of saying I have no idea what your problem is, but these are some areas I would look into.
By the way, root-balls and their multi-directional grain makes some of the most beautiful wood. I harvest my own trees so when it comes to Black Cherry, I saw my tree right at ground level. That first foot of so of stump has the most beautiful curly cherry you have ever seen, and every Black Cherry tree has it, most loggers just don't cut the tree low enough to get it.
