It's not possible to say which problem you might have, without having a look at your system. I have way better fps and smoothness with P3D4 than FSX, on the same system, which IS average for today's standards ( i7 6700 + GTX980 ).
The main difference between FSX and P3D4 is that, with FSX, you can add better hardware (graphic and, especially, memory), and you won't see much difference, while with P3D4, you will get visible benefits improving the hardware.
Since you WILL update your hardware, sooner or later, it means that with FSX there's no future, since the sim is stuck in 32 bit forever, so you WILL switch to P3D4 entirely.
While you, as an user, can say "I'm not switching to P3D4 *now*", we cannot afford that luxury, because it takes at least a year and usually more, to do the kind of detailed scenery we do, which means we cannot design a scenery for the PC/sim you use NOW, but we should anticipate what you'll be using in the 2 years.
Because, of course, when you will switch in 2 years (and you will), you wouldn't want to buy stuff that was made for the 11 years old sim...