He who is Carlos E.R. said on Tue, 26 Dec 2017 20:33:49 +0100:

Almost every system upgrade of any operating system makes the machine
slower, simply because they have new features, they are bigger, use more
memory, and sometimes they run more default tasks on background. So it
is expected and normal that after an upgrade machines feel slower.
Drastically slower, no.

Sometimes they run faster because they managed to optimize things.

You Apple Apologists are funny people.
Have you even looked at the SIZE of the slowdown?
It's drastic. It's huge. It's phenomenally large.

Even Apple has admitted it.

Apple secretly, drastically, and permanently slows down phone CPUs with
each iOS update.

The best solution is to do what I've always done.
Don't update to the next iOS release.

That's the best speed trick ever.