In 2008, airlines began charging for checked bags[1]. This was done both for the immediate revenue increase, and also to prod flyers into airline loyalty programs or airline credit cards to get a free checked bag. However, that caused a lot of casual fliers to go carryon-only. That, in turn, causes it to take longer to board/exit planes, leading to longer turn around times.
I've long contended that airlines should get rid of checked bag fees. And if they feel like they really want to be evil, switch the fees to carryons. That would decrease the number of carryons and decrease the turnaround time.
EDIT: From the article "Starting around 2008, Scheduled flight times began increasing even faster than actual ones" This has me convinced that the bag fees really torpedoed turnaround times.
[1] https://www.farecompare.com/travel-advice/airline-fees-bags-...