Either one could be getting stuck. You'll have to look & see. Also might be sticking inside the cable itself. Maybe even a pivot pin messed up on the pedal itself. Or a broken return spring on the throttlebody? Unhook the cable at each end, so you can work each part individually. Sometimes that's the only way to figure out which part is NOT moving freely.

Whatever's dirty, for sure clean it up. One of the more common places for it to stick is where the butterfly *almost* touches the wall inside the throttlebody. But that would usually stick closed, not open.