Chris I think you are very right to point this out. Whilst it wouldn’t stop me from buying the plus which I own (the obvious being the screen estate bonus), it has obvious bugs. The phone is so much more powerful than its predecessors and the features won’t be the thing slowing it down. It has to be attention to code like you say.
Recently my experience has been less than it should be with jarring on frames….safari has a glitch every so often and iTunes loves to play up sometimes instead of playing.
iTunes really needs sorting out – In implementing music, it’s neglected its core music player. I can’t even readily see things like how many albums I have (without going into settings). And the fact it keeps prompting me to join Apple Music is quite annoying (and takes time every time to load up).