Hi Beppe, Aoktar. I already wrote earlier about this issue.
Problem with increasing motion blur calculation. The problem may not appear from the first frame.
In this scene, it appears from about 70 frames, but somehow manifests itself on any scene with a large number of particles and motion blur.
Rendertime increasing frame by frame with same number of particles in each frame. Next, I restart render and rendertime on same frame getting much smaller and starts growing again.
It's very important problem. It makes impossible to render animations with a large number of particles.
Scene file. You need to cache simulation before render.
https://drive.google.com/open?id=1dlOOW ... ZHNKsWcHs7