Another thing not to gawk at is reducing the pixel size of the images used to generate the effect.
you definitely do not need a 2k texture to render a 10px wide image.
Changing the size may not be a lot of savings, but its still savings.
That said, the drop in fps must be a result of (or a combination) of draw calls and shader instructions.
meaning. How many particles does the smoke use? Can that be reduced? Are they GPU particles? Same for explosions.
Also, assuming that your system uses Cascade, maybe making a new one in niagara could lead to a more performant effect .