My iOS device is noticeably laggier too, and I find it difficult to point the finger at security fixes. More likely the bloat from a load of features that nobody asked for.

Because devs live and breath security. End result is that they develop blinders to anything else.

Also they know when and why to update stuff they work on, thus i never happens at an inopportune time. Nor do they find themselves having to wrangle the new dependency kudzu they created, because for them it built up over time.

I don't think it's the added security, that makes new software slower and less responsive.