There’s something else going on - my daughter has an iPhone 6 (is a hand-me-down of a hand-me-down). It is not appreciably slower than my wife’s 6+, and only slightly noticeably slower than my iPhone 7+.
EVERY computing device seems to slow down over time, whether that be a smartphone, a tablet, a laptop, a desktop, or even a mainframe computer. As code becomes fragmented on the drive (even solid state drives suffer from fragmentation), and as the actual memory capacity is filled up - the nature of the beast is that it slows down in dealing with the additional overhead.
Throw in that not only new and updated OS versions add additional burdens to the finite resources, but apps continue to evolve (and add bloat) which further task the various components, and it is unavoidable to see a measurable “slowdown” over time (even if that is more perception than reality when compared to newer devices).
Not to throw a straw man into this, but I’ve never seen a device from any maker, particularly Android-powered, that didn’t slow down - and indeed, Android often sees crashes and even malware to further tax the device and user.
But back to your particular situation - again, in a side-by-side comparison, running the same OS, there is very little appreciable difference between my daughter’s iPhone6 and my iPhone 7+ in app opening, and general tasks. And right now, there is actually a bit of advantage on her iPhone 6 - as I am in the beta program for iOS with my iPhone... so it it still needs optimization.
40 years in computere - you are correct.
The slowdown on mine was sudden, shocking, & the immediate result of installing IOS 11. Aps that would load in a second or two suddenly were taking 15 - 25 seconds to open.