I have heard similar complaints about performance from my friends, and I have solved most of them by simply suggesting they check their fans. Not just case fans, but the GPU/CPU, too. Check that they’re actually spinning and getting adequate airflow. Unplug and then vacuum them. You’d be surprised how much of a difference that can make.
No not use any DLC equipment. While the combat in the game is far from perfect, using the viper gear (I think that’s what it’s called) ruins it further.