You are probably right

You also need to clean lint out of the dryer itself that does not get into the lint filter. On some dryers, the front will come off easily for you to clean. Anytime I notice the dryer not drying the clothes as well as it should, I clean lint that does not make it to the lint filter and the vent.

Get ready to replace the thermal overload switch on it. It's about burnt up by now. Air flow restrictions eat them like candy, eventually.