It's amazing that we've found that Headspace is the common thing between us on this issue. I thought it might be a bad update to Headspace, but I just checked the App Store and Headspace hasn't been updated since November 20th. So I think a previous poster was right when they said that it's a combination of Headspace + the date + notifications. I don't think it's to do with the phone's time/date *in its current location*, but more to do with the date somewhere else (like the date/time-zone of Apple's notification servers). The reason I say this is because I am in New Zealand and this problem started occurring for me around 00:30 (just after midnight) on Saturday 2nd December. Everyone else is saying it started for them on 1st December, but I'm assuming most of you are on other time-zones. That's why I think the 'trigger' or cause of this issue isn't the app itself (it hasn't been updated by the developers since Nov 20, 2017) but something to do with the rollover to 1st Dec, and something to do with notifications.
By the way, I had a weird hiccup issue with this. My last (big) post said that I was going to install apps one by one - the apps I'd managed to hit pause on, before they finished downloading/installing. I unpaused Headspace, the issue returned, I had one twirly-wheel 'reset', so I deleted Headspace, and I actually had another twirly-wheel reset. Just as I was thinking "Oh, MAN, this makes no sense.. what now?" - I waited another 15 minutes and I had no more resets. That was about 20 hours ago and the phone has been fine since. In fact it's definitely running faster/smoother than it ever was, it's been jerky and horrible since iOS 11 was released (and continued that way, no matter what updates Apple released). Just dumb things like slower/jerkier unlock/home-screen animation, sometimes jerky keyboard response, or jerky animation when flipping between open apps. Now it feels as fast as it's ever felt, and I'm not just imagining this - it's way way better. I don't know if that's from uninstalling Headspace, or if it's because of iTunes Restore then restoring from backup, maybe something got cleared out.
I hope the developers of Headspace are reading this, because it seems like something's up. I don't want to pin the blame on them, as for all we know, it could be an issue with Apple's push notification system, combined with date rollover, etc.
Just looking at someone else's comment that iOS 11.2 just got released and it fixes this bug, where did you get that information from? Looking at the release notes for 11.2, there's no mention of this bug at all. I would be surprised if a major release like 11.2 could have a fix for this bug built into it, unless Apple knew about this bug before it became symptomatic, and quietly fixed it without saying anything. Point releases are a big deal and take time to develop. Quickly-developed bug-fix releases are typically a point-point release.
I'm going to update to 11.2 now though, and see if the issue comes back when I re-install Headspace.