Device: iPhone 6 128GB
OS: iOS 10 (latest available build at time of writing)
Usage pattern: almost exclusively music, syncing things at "High" quality
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I've been struggling with sync speeds on iOS for a while, and it's really quite frustrating (iPhone 6, iOS 10). I'm sure part of my problem is due to having an older phone (~2.5 years at time of writing), but I also happened to grab syssecinfo when it was available, so I have some insight into what's happening.
Whenever I'm performing a sync operation, CPU usage on my iPhone is at 100%. Maybe it will dip down to 96% or so, but it's close enough to 100% that it's not worth distinguishing. I don't know why the CPU usage is so high, but I've run sync operations for long enough that my phone has become uncomfortably warm.
Outside of syncing, Plex is generally just incredibly slow to use: it feels very much like it's running a full Plex Server instance on the phone and talking to it via a wireless network, rather than just accessing local content (my mobile bill disagrees with this statement, but that's what it feels like).
I was under the impression that all the transcoding was happening server-side and my local device was just pulling down the transcoded content. So perhaps its a writing-to-storage thing? Perhaps its doing something weird in the local network? I don't know, but whatever is going on makes my whole phone unusable whenever I'm syncing content -- and using Plex is an increasingly frustrating experience.