It was strange that yesterday, the iPhone worked really slowly. The screen can freeze for a few seconds. It was like the Processor or the graphics chip went bad or something.
It was like that when playing Pokemon Go, or just using iOS and the Settings as well, and it was the same even when I restarted the iPhone and used the Settings app -- it was really slow.
And I checked the free storage left, and it was 1.9GB. So I thought it should be plenty. In the past I have 500MB left and iOS was running all fine.
But this time, it is the iPhone 6 Plus, with 128GB storage, and 1GB of RAM, as opposed to in the past, when my iPhone 5 was just 16GB and it was common to have just 2GB free remaining.
And turning off the App Refresh and restarting the iPhone didn't help either.
So I deleted some big apps like Angry Birds and PvZ 2, which I play on the iPad instead of the iPhone anyway. And when it was 3GB, everything ran smoothly.
And since then, I uploaded some photos to Google Photos and freed up about 2GB of space last night, and made the free space about 5GB, and everything was running really well.
So 1.9GB seemed like not enough and 3GB was enough... what is the amount of free storage that should be kept for iOS itself to work properly?