That doesn't really have a lot to do with it.
I'll write that again.
Any sandbox game, where the amount of stuff going on can't be controlled or constrained by the developers, will always show framerate dips.
You can design your game to only use 50% of the GPU in typical situations, so that when things get a lot busier than expected, with the player mowing through hundreds of zombies in a flame throwing steamroller (with shadows being cast accurately by multiple dynamic light sources like headlights and beacons) you still have a lot of spare power to handle things without your framerate dropping.
Or you can have a game that uses 80% of your resources in typical conditions and be able to maintain 30fps for 'a lot of the time' but still suffer from framerate dips when things get extremely messy.
Point is the huge 'variability' in the amount of stuff that can be going on at once in DR.
Either you 'underuse' your hardware a lot of the time, or you risk a few slowdowns.
The point is striking a balance, and from what i've seen, things are not bad at all in DR3.
Dynamic resolution wouldn't have been a bad idea