I would look to some other cause for the slow down. Start with effects.
I haven't changed the effects I use, the type of videos I make, or anything other than the GPU.
I'm making identical content to what I was before the GPU upgrade.
It looks like Premiere Pro is just now underutilising my GPU, like I said, it used to take A LOT of work off the CPU to the point where my CPU was only under about 50% load but with the GTX 1080 my CPU maxes out and the GPU barely gets used at all (10%).
So is there another bottleneck? What operating system are you using? If Windows 8.1 or 10, the Resource Monitor will show you what is going on.
You can run my standardized hardware Premiere Pro BenchMark (PPBM) and watch the usage of the CPU and the GPU intensive export those two tests (out of the four) always show 100% utilization of the device under test. The scores from all four tests can also help find any weakness in your computer.
1. New graphic card is much more powerful and simply does with 10% what previous did with 100%, thus choking the CPU... maybe CPU options need to be dialed in to something more appropriate... memory options?
2. New graphic card is not too well supported by Premiere... drivers? Old drivers?
Good luck in troubleshooting!