Are you running the latest official patch or the more recent (but not finalised) test patch? The first one does have some problems and the second hasn't optimized the issue yet (it mainly deals with curing the CTD problem).
In any case, particles do cause an FPS hit and it's one of the things the developers know about. Going by what they told us in one of the recent updates, they are currently working on optimizing grass, trees and dust/smoke.
So, we expect that the next testing patch will have an optimization for this problem.
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