I signed up just to help you. I've had this problem too, and here's what I've tried and some stuff that I think will help ya....
Disk Defragmenter- Maybe you downloaded some files that fragmented your disk drive, you should do this and it helped run my comp much smoother.
If you're using a laptop, I notice that if you don't plug in the charger, the computer's performance decreases drastically possibly to save up energy. So plug in the charger if you haven't! Another tip: Having the computer on full performance got my fps from 40 to 60 for me, and not keep it on energy saver mode or wtvr. Sometimes computers may also overheat and dirt may be clogged up in the fan or something. I'm pretty sure there's a thread on that last one, a good one too, I hope someone can find it, but good luck and hopes it helps you!