arcturuz
Greater Supremacy Member
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from a post over at macrumors
i used to work in software engineering at apple so i can share some insight here. (i don't work there anymore otherwise i wouldn't be allowed to speak at all on apple related forum regardless of topic.)
when you upgrade to a new version of the operating system, in order for customers to use their phones right away (instead of seeing a blank "setup" screen), we continue a lot of optimization, database migration/upgrade, etc behind the scene. some of these things will be done opportunistically (e.g. when you are charging overnight), but some activities need to start right away. these background activities will keep the ap (application processor, aka "cpu") on in the background, competing for the resource your foreground applications are using.
give it a few days and you will see a much more stable stead state experience.
however, as any major version upgrade, performance, battery life, and stability will suffer quite a bit in exchange for features. how apple (and most companies) releases software is the major (X.0) versions are for features, and subsequent versions (.X) are for performance/battery life optimization, bug fixes, etc.
trust me, there are teams of engineers whose jobs are solely dedicated to improve your phone's performance, reduce crashes, etc.

