iMaxDesired sets the maximum amount of particles "desired" to be rendered. This affects smoke, sparks, lights, and similar effects. The way it is designed, if set to 0, most of the time particles will not be seen, but sometimes they will "flare up," hence the "desired" part. This can be safely lowered to around 250 without much visible effect, but will not improve performance. Indeed, in testing, higher values tended to produce smoother frame rates. However, it may prove useful in reducing the amount of flickering that objects emitting a large number of particle effects are prone to in certain circumstances. Extremely high values can cause some windy snowstorms to become a little too severe. If increasing it to max it out, aim for around 10000 (this number was arrived at by the misty, snowy conditions atop the Throat of the World).
Default is 750.