It always like this since first W10M insider build, I believe there is some kind like Realtime Report Activity, but not that active like in desktop. I've check the report only report some Apps open/closing/crashing events (like Photos, Messaging, Phone, Outlook) and Settings. It only send report if we interact with it, not in idle or reporting cpu usage I think.
But the main problem is battery saver is never really work, not the "disable background task if below 20%" thing, but the background task option to turn on/off of which apps can run in background, I tried to disable BG task for Outlook, and many others app like WMPU & NPU app, eventually those app does can run in background, still show notification and download in background. So the heating issue mainly comes from OS that can't really control the BG task.
And checking software update sometimes run every 2 hours.
My Lumia 620 only last about 6 hours in W10M compared to WP8.1 up to 24 hours.