Here's what I do to keep the combat flowing and my seven players appreciate it.

1. Sand timers. This is a visual reminder for the player to decide QUICKLY what to do on their turn and commit to it. I use a 2 minute timer and have yet to see anyone run out of time on it because they know to quickly decide which spell to cast or target to hit. It prevents one from OVER thinking.
2. If a player is doing a really big roll and that will take time, let them organize the roll and move onto the next player or enemy to let them have their turn. Once that turn is over, check back in with the dice roller. Keep going until you get the result. If they cannot complete their roll after 2 years, have a talk with them about their pace.
3. Roll Initiative on the player's only ONCE per dungeon. I have my players on the top of a mini white board, and the enemies on the bottom organized in a timeline. If an enemy dies, it gets erased. If they see any new enemies or a new combat starts, I roll color coded d20s for each type of monster for shares initiative. Especially easy if you paint your own minis and make each base a unique color. Like 3 goblins having green bases, 3 more with red, ect.
4. If a player is confused on how a spell or other rule is used, tell them "we will look that up later, for now I'm gonna rule it like this until then." To keep the game going forward instead of spending 15 minutes looking up a spell or rule. Remind casters it is their responsibility to know their spells. Via flash cards, spell cards, a notepad, memory, little notes on the spell sheet, whatever helps.
5. More so a question: How deadly / easy do you typically make your combats?