It happens fairly often in Houston (despite FAs asking passengers to close window shades before deplaning). Usually they hold up boarding and when we got on it still feels like its 100 degrees. It probaly isn't, I sweat plenty at 80. But a hot plane is a legitimate reason to wait until the last possible moment to start boarding in my opinion, especially if its a light load and won't take the full 35 minutes (or 20 for Express or 45 for bigger planes) to board.