I'm looking for mechanical options that might speed up the time it takes to get through combats. The issue is not how long the players take on their turns. The issue is not looking up rules. The issue is not the battlefield grid we use or the miniatures and how they move. The issue is not tracking initiative. The issue is not the number of combats per game session. I have received lots of advice about these issues. As I stated previously, it's all great advice but doesn't help, as these are all things that have already been streamlined.
I think the issue is that the party (made up of an evocation wizard, a cleric of war, a gnomish warlock and a paladin of vengeance) could be falling behind in their damage dealing somehow. They don't hold back their big attacks, so I know it's not a matter of them only poking enemies with cantrips (though the warlock does use cantrips frequently, but I think that's to be expected). There are plenty of evocation spells flying about, smites going off and the cleric of war splitting his time between cleaving things with his sword and inflicting wounds upon enemies.
A recent battle had them squaring off against two shambling mounds (around 136 hit points apiece) and a cleric of Moander (who had about 70 hit points). This battle somehow took around an hour. This isn't even a big boss battle or anything (the group is attempting to fight their way into the Pit of Moander in the Ruins of Yulash in the Forgotten Realms). And it wasn't due to the fire resistance of the shambling mounds, as the evocation wizard has the ability to bypass it completely. So, I'm just confused.
The battles don't always take forever, though. I threw lower level enemies at them prior to them entering Yulash. They obliterated the opposition in the first round before the enemies even went. Combat over in less than five minutes. So, it's quite jarring to go from five to ten minute combats to things that can take over an hour due to hit point inflation.
I mean, I expect there to be some increase in time, but not that much. So, any mechanical advice on how I can keep things up to speed while still keeping the dangerous enemies deadly?