Video Card for that system is crippling the performance. CPU and RAM is plenty for most games out there so no motherboard upgrade required. I'd invest into a better video card than the GT 520 which is weak in todays standards.
Also if you want speed, you can add a SSD drive to the system and have that as your OS drive, and if you get one that is say 128GB or larger you can even have games installed to that drive for fast load times.
I had added SSD drives to my older systems and it noticeably increased performance with faster boot to desktop and games launch much faster that are located on the SSD.
The biggest thing slowing down that system is that you need a better video card. I wouldnt throw any money into motherboard, CPU, or RAM as for they are still a good match for a modern gaming rig with the Core i7.
Lastly system lag has a tendency to build over time with systems in which a system that hasnt been rebuilt in over a year that has had software added/removed etc will start to become inefficient. So backing up your data to an external drive and then wiping the system clean and rebuilding from scratch can make a laggy system run like new again. Which reminds me that I should actually rebuild this one system that I am on now because it hasnt been rebuilt in about 18 months and lots of games and software added/removed and its not as peppy as it was when clean build. This time around with the clean build though I am going to create an image of my system to restore it back to a clean state so that next time I wipe it clean, i wont have to go through the manual rebuild process.
