I use a 5s as well. I cracked the screen on my first one but I'll be replacing it to use the phone as a hardware key soon. The only time I suffer lag is someti

I use a 5s as well. I cracked the screen on my first one but I'll be replacing it to use the phone as a hardware key soon. The only time I suffer lag is sometimes with the share button in Safari. My storage was chronically full on my last one, and that didn't seem to affect anything. My batteries still last a handful of hours, but the 5s wasn't part of the iOS voltage-limiting patches, so I don't know if that would be a cause. I'd try a factory reset then restoring from backup.