It looks like you have already put a lot of testing into your problem. If you have already narrowed down the issue to just swapping out the graphics card and the performance dropping for 2014 then I definitely believe you have found the problem. The unfortunate nature of new hardware is that, while more powerful, it doesn't often come with full functionality for older software. Your card was launched in February 2017; just over 2 years after SOLIDWORKS 2014 stopped being developed. I'm saying all this because, while I hope you find a solution to your performance issues, I don't believe that one with be very forthcoming.
If your Flow Simulation software is stuck in version 2014 then maybe the only viable solution is to remain on the older card for the sake of that performance.