A small note concerning the performance impact of weather generators: usually, the FPS hit comes from the fact that these programs generate much more clouds than the default real weather did.
There are however, two ways to limit that FPS impact:
1- In some programs, like Active Sky Next or Active Sky 2016 (which is the best of them all), you can configure the maximum amount of generated cloud layers. By default, it is set to 5, which was too much for my old computer. I limited this to 3 only, and I got an excellent weather rendering coupled with almost no FPS hit at all
2- Many people tend to forget the default FSX clouds are both ugly and heavy on the performance. I'd recommend them to download and install the freeware clouds by Pablo Diaz, named HDEv2. By choosing the cumulus textures in 512x512 DXT5 format, the clouds will not only look greater than the default, but their FPS hit will also be lower. This is very sensible in heavy/bad weather conditions, you can really see the difference. I know 512x512 sounds like very low resolution, but I assure you the visual quality is so much better than any default cloud. In fact, I'm still using these clouds in my P3Dv4 today. And they look magnificient

Bonus: some people chose to activate the Sparse Grid Super Sampling (SGSS) antialiasing mode on their NVidia video cards, because it is more or less the only way to obtain a decent antialiasing in DX10 preview mode in FSX. Unfortunately, this kind of antialiasing will kill the FPS each time a transparent object occupies too much space on your screen. This is valid for clouds, but also engine smoke, etc... Disabling this antialiasing mode (done through NVidia Inspector) is a good way to improve the performance in bad weather, although the antialiasing quality will become much worse...