Are you using a HDD or SSD for game storage? What are your RAM speeds? Either one could potentially be a bottleneck. Also, did you OC your 2500k? Sometimes too agressive of an overclock can actually perform less if it's unstable.
If you want to upgrade the CPU you'd also need a new motherboard, you'll also need DDR4 ram if you're going Skylake or Kabylake or something like that so it'll be pretty expensive.