If you're sure you've got everything set to low, I'd say your gpu needs an update. Not sure you cpu would be an issue, but may be a small part. I would think the gpu is mostly the problem. What resolution are you playing at? try dropping it as a test.
launch and leave task manager open while you run the game for a few minutes as is, then drop the resolution and see how the cpu usage changes. how close are you to 100% usage during both tests. what percentage difference does there seem to be?