I discovered that BT, our ISP, were offering McAfee as an extra included in our package and so I downloaded it and installed it on a Windows 10 laptop and on my iMac.
I initiated full scans on both machines last Thursday (8 September) and within a short time the scan was completed on the laptop (which has been little used), with no threats found. The scan on the iMac still hasn't finished (and I haven't closed the computer down for the whole week it's been running)!
So far it has found 165 threats, it says, and scanned over 57 million items. (Please see screenshot.)
I have tried to expand the filenames in "Currently scanning" without success, but am really not sure what I could do with these names even if I did get them. Unsurprisingly, what I can pick up of the filenames is a mixture of things I know are on the computer (and, as the beginning of the filenames indicates, the external hard disk) and the weird gobbledegooky names of bits of software (good and bad).
I generally run the Mac without AV, partly because many of my unofficial mentors recommend that for Macs, and partly because several years ago, when I was running Sophos (I think) and the computer had some problem (running slowly perhaps?), an adviser, probably from Apple Support, suggested that the Sophos was the cause of the problem. (Actually, I suspect that my machine just IS slow. It is my first Mac and I still love it, but despite lots of advice from Apple and other people it has always been disappointingly slow. But, as I said, in spite of that I love it.)
Before BT offered this McAfee program, I felt that my beloved slowcoach was performing extra slowly, so - perhaps unwisely, and without giving it a lot of thought - I took the McAfee route.
BUT, for the past week, while this marathon scan has been taking place, the slowness has reached new depths, absolutely EVERYTHING - eg, opening a new document in Word - has taken MUCH longer than usual.