Multithreading is the capacity (and necessity) of modern CPUs and Operating Systems to run many pieces of software code simultaneously in parallel. This contrasts with an old OS like MS-DOS and early generation CPUs, which ran a single thread of code instructions. My simple XP netbook is running about 500 threads at the moment. A thread scheduler determines the way the threads are executed so that everything gets done and things happen when they should.
More processors or more cores gives the scheduler more choices of available execution units to run code, making the process easier to organize and more efficient. And more physical execution units means more can be happening at the same instant, which is beneficial for time-critical functions like generating multi-track music with effects algorithms, synthesis etc.
In FL, depending which version you're using, check menu options for any reference to multithreading and ensure they are enabled, things like Mutlithreaded Mixer processing.
I mistyped in my previous mention of the distinction of i5 processors not implementing hyperthreading. This is a way to provide a somewhat similar effect to having twice as many cores. Each core can simultaneously self-manage two threads - there appears to be twice as many cores available and offloads some of the scheduler's workload, potentially increasing efficiency. It's a bit of a trap that desktop i5 CPUs may not implement hyperthreading (my laptop i5 does), meaning an i7 optimizes running software that responds well to hyperthreading. It might be a bit hard to be sure exactly which software this most applies to though!
Regardless of how significant multithreading is to any individual application you are running, Windows itself is massively multithreaded, and benefits significantly from multicore CPUs.