Very Slow broadband completly specific to iMac
My late 2012 iMac running Catalina 10.15.7 (the latest version available) has very slow broadband download speed.
It is the iMac, not the network. All phones iPads, MacBook and windows devices receive about 70 Mb/s when placed immediately next to the iMac.
The iMac receives at about 3 or 4 Mb/s whether it is on wifi or ethernet. Curiously it has the same upload speed of about 12 Mb/s as everything else.
I have downloaded a copy of Crome specifically to see it it was a Safari issue - It isn't, Crome shows exactly the same speeds.
As The copy of Chrome was downloaded just for this purpose it was clean with nothing in any caches or history, so at least I have discounted Safari itself as the cause of the problem.
I am guessing there is some setting, or such like, that is wrong, but I haven't a clue what it may be.
Any ideas what I should be looking for, gratefully received.
Many thanks,
David.
iMac 21.5″, macOS 10.15