How much AI traffic are you using?
What weather conditions are set?
Some scenery are not what you would call frame rate friendly, it would depend on how the scenery was made.
I would suggest you contact the author of the scenery to see if this is the case. Other than that I would try it with no weather set and no AI traffic and as simple model airplane you can get like the default Cessna If the the frame rates do not improve a whole lot you will be left with a decision. Do you really want this scenery if so you might have to live with the lousy frame rates.
Your choice.
