It seems like BP actors hidden by max draw distance set in the details tab (blueprints editor) still hit FPS as if they were still visible. As a test, I filled a scene with the default fire particle system (just spam hundreds within view). That cause FPS to drop to 17 when all are visible. If I toggle them to not be visible on the checkbox, FPS stays well above 40. However, if they are hidden due to max draw distance, even with some low number like 100 that makes them all hidden so none show on screen, FPS still drops to 17.
I updated this question to broadly apply to BP actors, as it seems like any component of the BP left visible (to be hidden by draw distance instead) affects FPS as if they were still visible the whole time. Previously I thought it was particle systems, but it also applies to pointlights, meshes, etc that are attached to the BP. For particle systems, it seems that using the particle system’s own LOD system is better than setting a max draw distance, as the former does not have a noticable frame drop while the latter does.