Earlier quoted context omitted.
Size is not speed.
True, but... Older hardware has older chips (and possibly slower memory) so... a larger size alone would still likely have an actual processing speed impact, no? The newer OS and app versions are developed with chip/memory speed "XYZ" in mind, and that's the target they aim for. That the OS does run on older hardware is great, but if your memory size goes up 2-3 times for apps, I can not imagine that there's 0 speed…
Apps are definitely bigger and slower, but that's separate from whether the OS is faster or slower.
Anecdotally (and I agree) iOS 11 is slower than 10 for many of the same tasks -- things like switching apps, opening the camera, opening the keyboard.