Any resistance type foot controller will heat up. The slower you sew, the hotter they will get.
Also if they are carboned up inside the carbon particles will cause shorting internally and that will make them get hot.
If the controller is getting hot enough you can smell it, that's too hot.
For FMQing or a lot of slow sewing it would probably be better to instal one of the electronic ones. From my experience they have great speed control and do not get hot.
Joe