Tried running Speedtest on the iMac: it wouldn't load the servers. Then I noticed an ad on the website for a macOS version from the Mac App Store. Installed that, it ran fine. I'm paying for 25 Mbps down & 5 Mbps up. The actual scores were 23.5 Mbps down & 4.97 Mbps up. I'm told by experts in the field that I can't expect better due to the type of connection: it uses new FTTN with the final 140 metres being telephone cable laid in 1980.