My D&D Combats are WAYYY too long. Help?

So as the title says my D&D combats are extremely long. I have been playing with this group for a while and honestly I have just gotten used to it, but now there getting so long, they become the focal point of the session.

For example. The most recent combat I had was against an Archangel boss. It was with 4 players (2 of them were out) and 1 enemy. The combat lasted 6 rounds in D&D but 2+ hours in real life. And while I know it was a boss fight and they were down two people of course its going to be longer. But another combat was a simple skirmish with some low level enemies, 4 people (again 2 were out) and 3-4 rounds of combat, and it was still 2 hours.

While I know some of this has to do with me having to improvise some of these fights, its a very open world game and I have really creative players, I know its not all of that. How can I reduce my combat times? Just a DM needing some DM advice.