The issue is not the game engine nor is it your hardware, Windows 10 has issues getting along with NVIDIA cards and is affecting a slew of games....one thing you can try that might help is to completely uninstall your graphics card driver, restart, and then let windows update find the driver....it likely won't be the latest driver and probably wont' include GeForce experience but for some people it is a version that at least works...
Tried this, still get anywhere from 10 to 15 fps drops during medium large fights. If it was Windows 10, then why do other, more intensive games run better? Virtually every other game I have tried has worked well. I even tried moving Sins to my SSD. No effect.
I had someone explain it to me, just wanted to let you know I have a solution. He said that some older games (especially games running on more outdated engines) just aren't fully compatible with newer drivers, which are optimized to run the latest 64 bit AAA title. Does that sound right to anyone here? It does explain my problem to the letter. I get great fps on Fallout 4 maxed out, but suffer with older RTS games like Sins and Total War: Rome II, both of which are notoriously unoptimized anyway.