Network access often very slow

I am finding that network access from my iMac is frequently very slow - download speeds of 1 or 2 kB/s are typical, which is obviously way out of line (I use istatMenus which shows network speed). This happens most of the time when doing general browsing, but occasionally I'll get lucky and get a 50kB speed. Large files, such as an OS update, usually come down at my ISP max of 8M once they get started.

While this is a nuisance anytime, where it is a real problem is doing a restart. It seems multiple online references happen when starting, and of course Safari reloads any pages which were open at shutdown, but this doesn't excuse the slow speed. A full restart takes at least 20 minutes, and I don't believe that I am doing anything particularly unusual (only a couple of startup items).

I'm sure it hasn't always been like this, but it has been for quite a while and I can't identify any particular event that started it.

Does anybody have any thoughts as to the kinds of things that might be happening, and what I should be looking at, to track down this annoying problem.

My computer is a 21.5" iMac (2011) running OS 10.13.6 (High Sierra - the latest it will support), with lots of free disc space and 12 GB memory and a i7 processor: there shouldn't be any shortage of resources.

Thanks for any help.

Paul.