FSX has always been a system hog. However with current hardware, I believe the performance has improved. I don't think it was that hugely un-optimized.
The question is do you want to pay for addons or get them for free? There are many great aircraft, scenery, etc both paid and free. Personally some of my favorite paid are
Flight 1''s Pilatus PC-12
Eaglesoft DG's entire line
PMDG's 747 and 737 lines
VRS F/A-18 "Superbug" (Truly a great product).
Carenado's Entire line (great GA planes)
Ultimate Traffic
Real Environment Extreme (REX) Great texture upgrade.