Soa is right, so it's a good bet it's not your rig. I get no lag Act I through V (sometimes a short spur of small lag if I am running something in the background, but that's to be expected), but in Atlantis content I lag practically everywhere.

Some suggestions have been to run in DX9 mode (if you're not already), and turning off HBAO+ or whatever it's called (I've read it's not optimized but I can't speak to that).

I was lucky in that my Atlantis lag wasn't enough to cause real problems, but it was just enough to be noticeable and sometimes mildly annoying. So I didn't try any suggestions when I played through it last time. Maybe try taking the hit to how pretty things look and just set everything way down when you're in Atlantis content, if you can stand it.

Hedgehog explains it well, but just want to drive home the point that you're going to really need a backup plan for constructs/devices. Starting off a vitality damage focused toon recently I quickly found I needed a backup weapon early on to deal with traps, mechanical/statue type enemies etc. Later on might be different but starting out you may want as good a melee or bow/thrown in your weapon swap. Look out for weapons that have a +damage to constructs those can really help.