I'm not aware of any fix for FPS. The game optimization is made for 25 FPS by Webzen. But yes, you can increase the FPS, but you will have the CPU usage eaten by the client.
Edit:
I've tried some fixes. Also, the game crash periodically and I suspect there are game clock dependent functions which are not running at an optimized game speed on my PC.
Usually, what I've noticed along the way is that the game engine being deprecated in front of the high end last generation video cards could cause trouble and FPS problems. I've tried to see the FPS difference between an Intel HD Graphics and RX480. RX480 - 19 FPS --- Intel HD Graphics 3000 - 24 FPS.
Also, what I think regarding this forever issue is that if we don't have the complete game client source code from the GUI drawing to last thing, I don't think there will be any fixes and I don't talk about DLL wrappers around the main.exe.
I'm gonna try the Vulkan Wrapper and see if there's any difference.