As others have said, it's not your rig it's SL. The engine is terrible and everything is unoptimized on top of the constant downloading of textures.
I have a fairly powerful rig and in some areas I'm lucky to get 10+ fps with both ALM and shadows off. This is a big part of the reason why I've switched to Singularity because it runs well just about anywhere compared to Firestorm where I can barely move in some places.
Compare that to Arkham Knight where I rarely drop below 45 on high settings and Skyrim where I can comfortably sit at 50+ on max settings+a very pretty ENB with like 10 dragons on the screen.