Data previews

Hi! I have some question, mostly to developers. I’ve looked at sources and look at UI_id_icon_render. So it used by bpy.ops.wm.previews_ensure(), but for python operator bool use_job is false. So can someone explain me why? For example if bpy.data.images contsins 200+ 4k images, simple calling previews_ensure via python freeze Blender for a few minutes but if I invoke icon render simply clicked to see list of images in the UI - this job running in background(thats much better). I tried to change use_job to true and everything work well.