Did you move from 5 to 8 on the same device (a big jump), or did you change devices? It may make a difference to the advice that can be offered. Also if it was the same device was it an official update or a custom ROM?
I moved from 5 to 8 last year, and yes, some things are different but the redesign of the battery settings is the only thing that I found really slower. But then I did change device, so expected changes, and don't have some of the issues you describe (such as the email problem).
Taking more time is often just that you have habits of usage that haven't adapted to the new system. To judge progress on that basis you should give the 2 versions to people who are unfamiliar with smartphones and see which they find easier - otherwise the old version will always seem quicker, at least initially.
On the keyboard issue all I can suggest is trying some third party keyboards and see whether there is one that works better for you. You never need to put up with manufacturer software that doesn't do what you want. I personally use third party apps for most things, which means that I don't suffer discontinuities if I update the OS or change device. Unfortunately I have never wanted to use a "phone pad" layout, so can't recommend an app for that.
If you changed from 5 to 8 on the same device, did you do a factory reset? It's a pain, but ifnyou didn't things like your email warning might be due to incompatible data from the old version. I'd certainly recommend going into recovery and clearing the system cache (if your recovery still has that option).
