Yeah, that's 100% my experience.I owned this one and considered it obsolete in 2014 when it was painfully slow with the latest ios version
I've had nothing but grief every single time I've upgraded iOS, on every device I've tried (iPhone 3G, 4, 6, iPad 2, iPad Air). Initially it looks great and shiny, but over time I notice the performance degrades so animations start to skip, the UI responsiveness seems to lag slightly. I'd guess it's due to increased memory footprint, but who knows? The Air was a real disappointment, it was so responsive before the upgrade, and was just that bit sluggish after.
So now I have a strict policy of NEVER upgrading to a new major version. My iPhone 8 is still perfectly fast, because it's still running the 'launch' iOS. I don't mind missing out on iOS features - can't think of any recent feature I need - but it eventually becomes a problem with apps. They'll only support the last 2 or so OS versions, so sooner or later they'll stop working.