I haven't read all 300+ posts in this thread but I did a search and I don't find anybody with quite the same combination of problems I'm suddenly having after a good 72 hours of normal operation of my new iPhone 8 (sigh).
I'm seeing 10-20 second slowdowns that are near constant. For instance, if I try swiping between my home screens I'll get 2-3 normally and then have to wait 20 seconds. Starting apps is a crapshoot. When they do start I often have to wait anywhere from 5-30 seconds for screen taps to register.
But, wait, there's more...
From the moment this started, I also lost haptic feedback from the home button. iOS seems to think it's set to the highest level, but, it just doesn't happen. Also, this morning, my alarm went off normally in that the clock app opened and would let me snooze, etc. It just didn't, you know, actually play the alarm sound to wake me up. Fortunately we're a multi-alarm household.
This morning I tried to do an encrypted backup and it failed twice (once setting a password for it, and once with an ominous "problem reading/writing to the iPhone" error before it worked on the third try.
I guess I'm going to do a restore and hope for the best. I've early-adopted the iPhone 3G, 4, 5, 6, 7 and now 8 with no previous problems. Maybe I'm just due. :|