I wonder how much of this is just caused by the higher workloads people put on their new phones just after purchase: restoring from backup, installing/trying new apps, testing out that new GPU, Photos doing a new classification pass on the existing library, and just a generally higher than average workload for the first few weeks of ownership.
It seems like there are always a flurry of reports of issues like this on launch week, which then pretty much disappear a few weeks later.