The maximum time a frame can spend on particle updates. If the frame takes longer than this, then updates are split into multiple smaller updates.
Use this function to balance the accuracy of particle simulation against your performance target.
Using a small value will give higher quality particle simulations, but will take more processing time. Particle updates will run multiple times at smaller time increments, if the frame time exceeds the threshold provided.
Conversely, a higher value will ensure that particle simulations are not broken down into multiple steps per frame, giving the best performance, but will lose simulation accuracy when using some of the more advanced particle simulation features.