Assuming we're talking about D&D 5e... this is a common issue.

More than 5 or 6 players will bog down combat exponentially.

Martial character options like Action Surge, Extra Attacks, and weaponized Bonus Actions will of course extend turn time. You can speed things up by telling the player the target AC up-front instead of the common guessing game: "does 14 hit? ok I'll use Inspiration to add d6, does 16 hit? Ok, next attack, does 18 hit...?"

Players being tuned out, needing a battle conditions summary in order to decide what to do each time their turn comes up... bad manners but are you really ready to tell an otherwise beloved friend to shape up or don't come back?

There's a fairly common condition, aphantasia, which is basically inability to imagine visually. Using a miniatures battlefield instead of theater-of-mind for more complex battles can help players with aphantasia stay on top of battle conditions.

Players who perpetually pitch outside-the-box, game-breaking interpretations of their abilities, like trying to use Acrobatics proficiency like a free Spider Climb spell... yep that taxes time even if the ruling is "no that ability doesn't work that way". Putting a time limit on each player's turn might help quash time-wasting spitballing.