I was probably a bit unclear in my post but I thought I made it clear when I noted that a healer cleric is extra-low damage.
The average DPR groups should average 5 or so rounds and the high DPR groups can easily be challenged with slightly tougher encounters to also do so.
Instead, an average DPR group takes a much longer time if the encounter is not same level. And n+2 encounter often takes 8 to 10 rounds or so.
In your 4 hours, you get 4 encounters. In our last two sessions of 5 hours, we got 3 encounters (n+2, n, n+1) two weeks ago and 2 encounters yesterday (n+3, n). Your average of encounters per hour appears to be about double ours.
And, that is to be expected for two reasons: 1) your group is higher DPR, 2) you have 6 PCs whereas we have 4 PCs. 6 PCs can gang up on a single NPC and change the action economy much faster than 4 PCs. 6 PCs where most of them have high DPR can do so even faster.
Now, a 4 person party is as much away from the norm as a 6 man party. A 6 man party has much more synergy than a 4 man party. In other words:
If you have fewer than 5 people and you are scaling the encounters by xp as the DMG says, in addition shave a little bit of the HP of the mobs to compensate for the lower synergy. 10-20% should be about right. In a larger party you can do the opposite, adding a small amount of hp.
One more thing is that combat speed in time also depends on your players. We have a very streamlined combat and people usually just act when it's their turn, instead of thinking... The same goes for me as a DM. We have had some funny situations because of it, but since it goes both ways it balances out nicely.
Our ranger for instance usually goes: "twin strike the ugly orc with the big rash. AC 25, AC 17, one hit, dmg 1d10+5+1d6 = 14". In other words his turn is over in like 30 seconds. Which is the right amount of time to use when twin striking.