Hopefully that 'secondary issue' you related here is something you may be
able to sort out by contacting Apple directly; seemed you may have gone
to an unofficial 'support' site where they gave you some un-apple advice.
{Your comment - and my reply from then - is gone; I reported post to Hosts
& hoped they'd look into this matter. ~ I thought you could've been victim of
non-Apple 'help' site; or perhaps given your AppleID password to phishers.}
Otherwise the inquiry first stated above appears to have a plausible answer.
There may be something with your wi-fi base station or source that is slower.
I've owned both 5C (recent to) SE iPhones, both with their latest supported iOS
with more than enough free capacities in their storage. And SE is generally fast.
..5C was fairly good & I didn't have issue with anything at all; but it's gone now..