I'm with you. Updated my 6+ and everything is tragically slow. 4-5 seconds just to get the Settings or Messages apps to open. Typing anything is unbearable - every time, the first word capitalises and then botches everything from then on. Basic functions within apps or opening the camera take so long as to render them near-useless.
I have switched off any animations and performance tweaks that I thought might help, but no luck.
If you have a 6, I'd recommend avoiding iOS 11 as much as you can.
iPhone 6 Plus here. Can confirm exact same issues.
I ended up backing up everything and reinstalling from fresh. It's only just bearable now. Still noticeable 2s+ delays to open iMessage & and to get into Settings. Using the device for me has become completely joyless. In fact I have incipient resentment just looking at this p.o.s. It was once a great device. It's now hamstrung with bloat. I have stopped using it much now - I haven't yet installed Twitter & GMail or any of the many apps I had again. An entire 25 GB cache of Music library has yet to be re-installed. The phone sits almost the entire day unused. Battery life at end at night for a recharge? 80%!
If Apple doesn't fix these issues quickly and restore confidence in the already dwindling user base, they will suffer greatly at the hands of the Huawei's of the market. You don't need a fascist type of Jobs to get these types of issues fixed.
I did some research and avoiding iOS 11 is exactly what I've done as there are too many unpleasant changes for me to want to upgrade yet. Besides, there are several apps I use that haven't put out updates to work with iOS 11.
I have switched off any animations and performance tweaks that I thought might help, but no luck.
If you have a 6, I'd recommend avoiding iOS 11 as much as you can.