My old iPad Air slowed a bit, but never into multiple tens of seconds. Maybe 3-4 seconds to enter multitasking view (which is bad enough, don't get me wrong).
What phone are you on? Have you tried a factory reset?
I've done a complete restore/set up new/bla bla bla... didn't do much. I have to either reboot or hard reset my phone almost every day because apps(even system apps, like messages) will either lock the entire phone up or just all start freezing signalling that's about to occur.
My phone is in perfect condition hardware wise, with great battery health etc(like over 90%). The battery runs down as fast as my 6 that had a defective battery apple replaced under that recall program. I was seriously debating keeping this phone for another six months at least, but if 11.1 doesn't fix it it's going on craigslist.
This is just hilariously inexcusable, it's almost ios 4 on the iphone 3G bad. Like my phone is pretty bad, but my roommates 6 will literally sit for yea, 10+ seconds waiting for the camera to actually be "ready". There's also random ENORMOUS lag when shooting photos/before tap to focus works/etc.
This really isn't progress. iOS11 introduced practically nothing in the way of improvements or new features, but instead has made a lot of devices practically unusable.