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One of the major CPU consumption cause of Stardust is the display object instantiation, since new display objects are created rather rapidly along the particle simulation. So I think using an object pool to store display objects that are no longer needed for future use is a nice way to improve performance.
The new PooledDisplayObjectClass and PooledDisplayObjectClass3D initializers are "pooled" versions of their counterparts, the DisplayObjectClass and DisplayObjectClass3D initializers, respectively.
I've run a quick performance test, and found that when using the PooledDisplayObjectClass initializer to initialize particles with display objects, the performance improves by approximately 10%~15%, compared to the original DisplayObjectClass initializer. It's not a significant improvement, but at least it's better :)