This is what my settings look like in FL right now. The big ones to pay attention to are probably "Multithreaded generator processing," "Multithreaded mixer processing," and Resampling quality. Bump it down to 6-point Hermite if needed, don't know if that's usually a default or not. You'll also benefit from getting a good external audio interface, as that will ease a bit of the load off your CPU for audio processing.
Also, check and see what your animation settings are at. Lower them if needed, as that will free up unneeded CPU usage.
Finally, unless you're using an SSD for your big samples, that's also probably a huge issue. Streaming stuff off a mechanical hard drive will bring even a supercomputer grinding to a halt, and it's generally better to run your DAW and your samples off separate drives. My C: drive is where I've installed FL Studio (and all my VST plugins), and it's a Samsung 840 SSD. That D: drive is a mechanical hard drive, so it only houses things like short samples (individual drums, breakbeats, etc.) and stems that don't get used often. The E: drive is where I've installed all my EWQL, Kontakt, Omnisphere, UVI, etc. libraries to. It's a Crucial MX200 SSD, and does a fine job keeping those libraries from being a bottleneck.