If you feed too slowly, the waste changes to powdery dust and there is some scorching along the cut surface.
Listen carefully to the sound of the motor. If you are feeding properly there will always be some slowing down, particularly with big cutters, but the sound must not become laboured. If it is slowing unduly, reduce the feed rate and/or the depth of cut in combination until the motor resumes a relatively free running sound.
All this assumes that the cutters are sharp in the first place. If they are not, then the feed rate has to be reduced further and the finish will never be of a high standard.