My dryer would not dry clothes unless it's set on high for 150 minutes. Used to take a lot less time to do the job. The vent has just been professionally cleaned. Problem started way before the cleaning. Can the heating coil be bad? Its a GE electric model. Thanks.

If you're sure the vent and filter are not clogged, I'd be more inclined to think it's the thermostat control that's bad than the heating element. Usually heating elements either work or don't.
Bit I would take the time to be SURE there's plenty of air flow out the vent before doing anything else.