It has something to do with the data interface setup in the generic simulation stages.
In UE5.2, you cannot longer select a dispatch type when you select Data Interface as Iteration Source (The Override Gpu Dispatch Type checkbox is gone).
Instead, there is now a new Iteration Source called “Direct Set”. Problem with Direct Set is, that you can’t specify a Data Interface anymore.
If you set everything to Data Interface in 5.2, you get something that looks mostly right but it’s extremely slow.
Update: The bug is that it ignores the “Override Gpu Dispatch Num Threads” setting in “Direct Set” mode in 5.2. I’ve made a bug report on UDN: https://udn.unrealengine.com/s/question/0D54z000096cseXCAQ/niagara-generic-simulation-stage-gpu-dispatch-override-is-being-ignored-in-direct-set-mode-in-52