Bear in mind, what the sim software does is take note of all the inputs, figure out what the next screen should look like, and send that info to the graphics card, at least 24 times a second for smooth motion.
That is why MS Flightsims are very CPU (processor) intensive. A good graphics card with a good GPU and lots of dedicated video memory will ensure you have no bottleneck after the CPU has done its thing; but no matter how good the card is, it can only process what it gets from the CPU - it won't make the CPU run any faster. When you have lots of AGN trees and buildings, and lots of AI traffic, each item will be in a slightly different position in the next frame, and all that has to be calculated; introducing a plane with complex systems on top of all that will drag most CPUs to their knees
One day, when the Toys-for-Granpa Fund is healthy again, I will have several SSDs to feed a very fast CPU, and a top-end graphics card to paint the screens very very quickly. I will only use the HDD for storing downloads and screenies.