Hiya!
I think the problem some folks are having understanding how a 5 hour session can get through, say 10 combat encounters boils down to two, 'simple' things: "RAW versus RAI", and "battlemaps".
I very much take the large scoop of RAI and slather it in "Just make it up and keep playing" sauce.
For example:
Player: "I rush towards him with my tower shield and try and push him back through the doorway!".
Me: "You have anything special to make that easier for you?"
Player: "Nope, just a big shield...oh, and about 40lb of armour on"
Me: "Ok. Athletics, Strength, Check against... DC 12"
Player: "Whoo-Hoo! I got a 17 total!"
Me: "You slam into the mercenary elf woman, easily sending her backwards through the door and onto her butt. You are now...right at the door? With your shield acting 'as the door' I take it?"
Player: "Ha! Yes. Exactly....you guys have a round or two to GET HIM OUT OF HERE! I'll hold the door!"
That is translated to ALL manner of rolls in the game. We don't do a lot of "looking up" of various rules and whatnot. We also don't use Multiclassing or Feats...and only use the PHB, MM, and DMG (with exceptions on case by case basis for other races, classes, spells, etc from other sources).
So, if you are running a game with "all the sweet, sweet candy sprinkles" (re: Feats, MC, all official WotC books, other 3rd party books, etc), and you are also trying to go by "the rules" (re: RAW) as much as possible....AND using miniatures or some kind of "battle map"? Then yeah...you're hooped!
Drop all that, go back to basics, switch to Theatre of the Mind, and I can all but guarantee you'll be finishing combats in a fraction of the time. If everyone is used to "visuals" and "all the sweet, sweet candy sprinkles" (rules/books), things may not go so smoothly for the first couple sessions as the Players grapple with the loss of "but look right HERE!..." (pointing at rules, miniatures, maps, etc), but they will eventually replace that with the wide-eyed wonder of imagination!
^_^
Paul L. Ming