Particle burst

I have a particle system for an explosion with 12 emitters. Most of them have many particle, 64 or 128, and, since the explosion is very quick I have to increase the spawn rates to 1000.0 (or higher).

I seems to me that playing this particle system tends to slow down the game.

Is this due to the high particle total count for the system or to the high spawn rate?

Related to this. It would be great to have a “burst mode” for an emitter; so that all the particles are emitted at once when the emitter starts to emit.