I also had Active Sky Next on my machine (test version), and it dropped the frame rates with all these Cumulus clouds. Aerosoft Airbus was close to unflyable (FPS around 5) and the default airbus was slow (FPS 10-15). I now use Global Real Weather, which does not create that much Cumulus stuff and looks a lot more realistic to me. Note that my system is on the weak side, and I have to be careful not to overload the graphics.
I did some tweaks to FSX.CFG too. I wonder, if you could post some details on this, what do do and what not to do. Many of the tweaks I took back after seeing that they do not work.