Rob
Ansys Employee

8M particles is a large number, so will cause some slow down. You're also calculating particles on some nodes: if they're all near the inlet you'll find they're all on one node. So, it's not particles being brought back to the host over particles swamping one node. How many cells have you got? 

If you want one pulse/band reduce the injection stop time to be 1-2 time steps. 

DPM time step is how long a flight time is modelled for, so 1e-7s in your case. The steps or length factor is then how often the position and vector are checked (minimum is some per cell) per time step. In your case I suspect some parcels are bouncing between very few cells and therefore using many steps over the track.