I have done a test for dealing with multi-million particles in Houdini.
In this test , I generated approximate over 30,000,000 (30Million) particles when pass to render , if you familiar with Krakatoa's workflow , this approach just like that.
(Separated into 5 partitions and echo partition contained around 5-6 Million particles.)
Here's no way to display Particles like these huge amount all at once on Houdini's viewport , so I only grabbed a screen for 4million particles .
All particles was genearted in POP ,not any dynamic simulation involved.
It seems need more higher substeps for generating more smooth-moving particles in POP .
Hope u like it and feel free to leave a message :)
Houdini's 30million Particles Test. One-5th parition