The repairman got here today, almost on time. I explained the problem and the fact that I had had the vent hose cleaned out about a month ago. He took the front panel off the dryer and I was shocked to find that it was full of lint. I now know that when the hose is cleaned, the front panel should also be removed for cleaning.
Then he went up in the attic to check the hose up there. It seemed that that was the problem. When the builder put the hole in the roof for the vent, he neglected to fully open the flap on the vent. The repairman fixed the flap and said that now the dryer should work perfectly.
It had worked so poorly before they put the hole in the roof that even the little improvement was appreciated. Today, after he fixed the flap I could feel the difference. The air in the dryer was actually hot. Now, maybe my clothes will dry in less than 2 or 3 hours.
At least I don't have to buy a new dryer - yet.