The default setting for most apps is to phone home every 15 minutes or at some absurdly-high interval. Once you lose 4G coverage your phone slows to a crawl. When you turn off automatic updates and notifications (which can be arduous or impossible for some apps) even older smartphones run well.
Every time my Samsung Galaxy S3 is running slowly, some app developer forgot my preferences and turned back on auto updates. The ABC News app was the latest violator.
There is no reason why an app can't load content on demand while running. When apps are not open, they should do doing nothing! Apps not being open can be a strong indication that you are not in a coverage area, or coverage is poor. So automatic updates when no apps are running is the opposite of what you want to do.
The only thing I want to be happening in the background is backing up my files, and even then only on wifi and while charging. If I'm doing nothing, apps should be doing nothing.