Is it the simpler the invention, the faster the approval / rejection decision?
The initial phase is queue time of about a year or year and a half. The art center it is assigned to has a small effect on this. Then the examiner searches and writes a first office action. They are on a quota system and can’t spend a lot of time. The content of the application affects this by days, not weeks.
That first office action is, statistically, probably a rejection. The next steps involve you responding by arguing or amending or quitting . Then you get another office action. You can fight or fold. Folding could be narrowing claims to the examiners liking and getting an allowance right away or it could be giving up.
There is no theoretical limit as to how many back and forth you engage in which will extend the time.