None of these answers help me. Not saying the advice is bad. The issue is not the players, as they are pretty quick in their decisions. The number of combats aren't the issue as they vary based upon what happens from session to session, and some sessions have none.
I have mostly noticed that enemies with more than 100 hit points take a while to kill. Maybe the party isn't doing as much damage as they should. Oh well.
Might we ask how long an average combat is?
I know for my group an average combat with nine players taking actions including two DM controlled NPCs and four monsters like Umber Hulks takes 5 to 10 minutes to run at level 4. A combat against 7 PCs and four mummies, one highly advanced CR 9 wraith, and 8 specters takes about 20 to 25 minutes. Are your combats taking longer than that? Shorter? Or about the same?