I don't know how your PC's blow through hit points so quickly. I hear lots of people talk about it, but I have yet to see it for myself, and I don't even see how it's mathematically possible without PC's critting almost every hit.
It's actually pretty easy to figure out the math.
10th level is a "weak" level for PCs overall. I took the 13 10th level standard monsters from the MM and here are their defenses:
HP 130; AC 22; Fortitude 21, Reflex 20, Will 18
HP 107; AC 24; Fortitude 23, Reflex 21, Will 19
HP 111; AC 26; Fortitude 24, Reflex 22, Will 21
HP 109; AC 27; Fortitude 26, Reflex 20, Will 21
HP 81; AC 24; Fortitude 21, Reflex 24, Will 23
HP 106; AC 24; Fortitude 23, Reflex 20, Will 21
HP 110; AC 22; Fortitude 23, Reflex 16, Will 19
HP 105; AC 24; Fortitude 20, Reflex 22, Will 19
HP 106; AC 26; Fortitude 27, Reflex 21, Will 23
HP 126; AC 23; Fortitude 22, Reflex 23, Will 20
HP 160; AC 24; Fortitude 21, Reflex 22, Will 23
HP 87; AC 27; Fortitude 25, Reflex 22, Will 21
HP 106; AC 24; Fortitude 24, Reflex 20, Will 19
So, the average same level 10th level foe would be:
HP 111; AC 24.4 (round to 24); Fortitude 23, Reflex 21, Will 20.5 (round to 21)
Let's use a party of Cleric, Fighter, Paladin, Ranger, Wizard. One of each role but Defender, two of those. We will only use At Will powers.
The 10th level Cleric uses Lance of Faith every round. Wisdom 20, +2 holy symbol vs. Reflex
The 10th level Fighter with Weapon Talent uses Reaping Strike every round. Strength 20, +2 longsword, Weapon Focus
The 10th level Paladin uses Holy Strike every round. Strength 20, Wisdom 14, +2 longsword, Weapon Focus
The 10th level Ranger uses Twin Strike every round. Dexterity 20, +2 longbow, Lethal Hunter
The 10th level Wizard uses Phantom Bolt every round. Intelligence 20, +2 staff vs. Will, Dark Fury
Except for the Cleric who probably doesn't focus on damage at all, I gave them all a single damage helping feat. The Defenders are sword and board, so no big weapon damage. None of these PCs have a +3 magic item (but some of them should), nor did they take superior weapons or implements.
Even without counting the increased damage of criticals, the Cleric does 6.9 dpr, the Fighter does 8.825 dpr, the Paladin does 8.7 dpr, the Ranger does 11.625 dpr, and the Wizard does 7.5 dpr.
This is a total of 43.55 dpr or 12.75 rounds to take out 5 such foes (worse case scenario).
Something as simple as counting the increased damage of criticals drops this by over a round or 11.5 rounds (~3 criticals in that timeframe). Combat Advantage a fourth of the time drops 1 round. Using 3 Encounter powers instead of 3 At Wills per PC drops it by at least 2 rounds and sometimes more. We're already around 8.5 or fewer rounds.
Optimizing the PCs more than I did, Weapon/Implement Expertise, racial abilities, the defenders getting an occasional free attack or damage, giving some of them +3 items instead of +2, throwing in multiple foe area effect powers, using two handed weapons or superior weapons, having more feats for more damage, getting power bonuses to hit or damage from other powers, dropping the mega 160 hit point outlier Leader monster from the averages, using focus fire in order to free up PCs to gain Combat Advantage via flank more often, hitting foes with vulnerabilities, doing ongoing damage, having two strikers in the group instead of two defenders.
It's not that hard to take this down to 6 rounds or fewer, especially with PCs that are just decently optimized.
I wonder why you don't see this in your games. Maybe you should keep a record of your encounters to find out how much and when damage occurs on the foes.