Michael Glenn:

Ever since iOS 11 came out I had experienced a significant performance issue on my iPhone 6S. Animations and transitions were slow, app loading was noticeably and unbearably slower than iOS 10 and my battery was draining faster. Many rumours swirled that others were experiencing this but not all.[…]

Then several weeks ago a Reddit conversation started spreading online that presented possible evidence Apple was reducing the performance of their iOS and possibly laptops when the battery life was sufficiently degraded. That day I decided to test the theory by getting my battery replaced at the Apple Store.

[…] After I confirmed with her that I was not using it that heavily and the battery setting statistics also didn’t show an application using a large percentage of the battery she suggested a rogue system process that somehow persisted through upgrades and restarts.

She also let me know that my battery was at 83% health and that Apple won’t even do a replacement unless it’s below 80%.

So I went home and immediately did a local backup, wipe and restore. And voila! Performance issues were gone.