There is a ratio option like filling rate on the actual emitter which tells the editor how many to draw max count. If the particles go above that count then the engine will stop rendering anything above that. ofc and someone correct me if i’m wrong this ratio depends on the users PC.
Try disabling from the actual emitter the draw max count or turn it to 0. Tell us if it’s sorted.
I would make very simple SubUV textures with low instructions (kinda too low for a next gen engine) and our testers would lag to oblivion while these fx play a bunch of times or when they overlap. No lag at distance so its definitely overdraw.
I asked on the aswerhub of you are keen to hopefully get feedback: