will always be the case when you upgrade to better graphics, without upgrading your video card as well, I have a GTX 660 ti currently and it runs at a solid 120fps with the “realtime” setting turned off, 60fps in realtime depending on how much is on screen is more than achievable in most cases, dipping to 40fps under load.
Unfortunately it comes with the territory when the visuals are upgraded, however a lot of these can be turned off or down in the viewport “Show” options, it depends on how good you want the graphics to be in your game, but having the “Quick Settings -> Engine Scalability Settings” all set to Epic and every other option on or on high, will of course slow you down without a high end graphics card (much better than mine) to achieve 120fps in realtime. 