MSFS is a very complex game, is currently not very optimized for various reasons (still using DX11) and thus can be difficult to run on even the best hardware. Your i5-2400S technically doesn't meet the minimum requirements listed for the game, which is a problem because this game is very heavy on the CPU.
Regardless of the GPU you upgrade to (which you obviously do need to upgrade in order to play this game) you are going to have poor performance due to being constantly CPU limited. The minimum required CPU for this game is an i5-4460, although the recommended CPU is listed a i5-8400, which is a 6 core CPU (yours is 4). I have an i7-9700k (8 cores) overclocked to 5.1 GHz and I still run into CPU limitations at places like LAX...