That assertion is so patently absurd that I can only contend that you're trolling. As such, this post will be my last that addresses anything you post in this thread.
A leader not only counters enemy debuffs which otherwise would slow down combats, but they provide their own consistent level of damage, provide buffs to party members to enhance their probability of hitting and damage output, as well as making sure characters don't fall and thus stop contributing to a combat encounter, but they also often offer out of turn attacks and other actions which bypass the regular action economy, thus again speeding up play.
Speeding up play and shortening the number of rounds a combat takes are not necessarily the same thing. In my experience, out of turn attacks, interrupts, reactions, etc., actually slow down play, while shortening the number of rounds.
Take for instance:
DM: Does 26 hit your AC?
Invoker: *sigh* Yes, just hits even with the bonus from the cleric.
DM: Okay, you take 24 points of fire damage.
Ranger: Wait! I want to Disrupting Strike that! *rolls* Uhh, lets see that would be a 24?
DM: That misses.
Ranger: Shoot, well uh, Elven Accuracy. *rolls* Dang! 22.
DM: Sorry, still misses.
Avenger: Wait, go ahead a reroll that since he's my Oath of Enmity.
Ranger: Sweet! *rolls* Woot! 28 will hit I'm sure! That will do 18 points of damage, oh wait, forgot my Quarry damage *rolls* that's another 6 points.
vs.
DM: Does 26 hit your AC?
Invoker: *sigh* Yes, just hits.
DM: Okay, take 24 points of fire damage.
Both versions have the same result essentially (24 points of damage) but one took a lot longer to get to while taking up the same amount of "game world" time. Defensive buffs/debuffs (i.e. buffing PC defenses and debuffing enemy attacks) do slow down combats because the PCs take less damage. Offensive buffs/debuffs (buffing PC attacks and debuffing enemy defenses) speed up combat by making the monsters easier to kill. Thing is, its not always guaranteed that one or both of those will be in place. Sometimes its better for the defensive buffs to be used, sometimes its better to go offensive.
Here's the thing though, combats without a leader can absolutely be very quick, it just might not be in the party's favor. Take a party of five strikers. They will deal a ton of damage in a hurry, but will also be killed in a hurry. Either way, its likely that the fight will be over relatively quickly.
Now a party with five shielding swordmages of course will have very long drawn out fights.
Obviously both of those are extremes. The point is though two people in this thread have experienced games without a leader and not noticed a drawing out of combat. I also believe that PCat's game has had long periods without a leader class and I don't recall them having too many problems. I won't dispute that you may have had different experiences, but don't assume that your experiences are the only correct ones and that everyone else must be lying if they claim to have had a different experience.
My question for you (and I don't mean this to be snarky, its an honest question) is do you consider your party and your players when designing your adventures? Your statements about strict rules as written seems to indicate that you design without taking the party/players into consideration. There's nothing wrong with this approach, but its also not a common approach in my opinion. Even so though, I haven't had to adjust much in my campaign that has been largely leaderless. The only changes I made were to healing potions to make them a little closer to what a leader would do (though not nearly as good). I don't use stun/dominate a lot but then, I wouldn't use them a lot even if the party did have one or more leader types. I just don't think they make for fun play in my opinion.