The performance of the particle system scales with the number of particles it is maintaining. After prototyping the desired effect, performance can be improved by lowering the particle count. Conversely, if performance is well within the acceptable bounds, you can increase the number of particles until you hit that point (should that improve the effect).
Note that particle count is often estimated by the particle system, and in some cases explicitly providing hints as to how many particles will be needed will improve performance. You can do this by setting maximumEmitted on an Emitter, and it is generally useful for Emitters which do not continuously emit particles.
Like ShaderEffect, the performance of the particle system is largely dependent on the graphics hardware it is running on. The exception to this is Affectors. For systems not including Affectors, the majority of the performance cost of particles will be on the GPU. Since the GPU is better at parallelizing large numbers of operations more particles can be drawn at 60FPS when Affectors are not used.