I feel the need to post an updated first impression because I hadn't given the phone enough time to settle and for me to get used to it.
The UI speed is fine, honestly. I think Spotlight may have caused the issues as Spotlight wouldn't work either and now it is: it was indexing everything on my phone. (I started from fresh, only have a few apps).
Gaming performance is amazing: games load fast, look sharp and the phone doesn't heat up, honest! ;o It stayed cool playing Asphalt 8 while charging and that amazed me...
Apps seem to load faster than on the 5s, albeit marginally at best. Twitter has a noticeable improvement for me. However, many apps haven't been updated for the screen and are simply scaled up iPhone 5 apps, which looks pretty awful especially with the huge keyboard within those apps. Also, performance in these scaled up apps seems really bad. They just need updating. Every Apple app looks great.
Safari performance is good, I haven't had any tabs reloading and scrolling is buttery smooth. The tab view is especially good, in landscape.
The form factor of the phone is great. It feels lighter than it is, I've been holding it for a few hours and my arm isn't aching. I can use it with one hand, and my hands aren't THAT big. However, I need two hands to type as my finger can't reach the 3 far left letters on the keyboard. The phone feels GREAT in the hand, it's so slim.
Any questions, please ask.
Also, battery life seems good. While playing on Beach Buggy Racer I unplugged the phone and it went down a % in 20 minutes