So I've been using this machine for a little over 10 years now and making upgrades as nessesary. I don't care much for graphic quality so it's mostly been fine. However, my FPS on certain games has started to become pretty frustrating so I'm trying to come up with inexpensive options to upgrade.

In particular I'm trying to play Nuclear Option online and am having a lot of trouble late in the match. I'm guessing this is due to the high amount of smoke effects but I'm not an expert in any way.

I'm getting 10~20 FPS regardless of graphic settings. I've also checked on my GPU and it maxes out at around 50% utilization so I'm guessing the processor is my limitation

The best CPU my motherboard can take is a i7-4790k (please correct me if I'm wrong) and I think I can get one for $100ish but I'm not sure if that would make enough of a difference. It also seems like DDR3 era board/CPU combos are a little more expensive but fairly affordable on ebay. However, I'm not really sure what would be a good value and worth the investment performance-wise.

Any advice you could give is appreciated. Thanks