Boot drive on 2017 iMac very slow

Hi all,

i noticed that my iMac felt quite slow (27 inch screen 2017 model with a 1TB hard drive), so I connected an SSD via USB3 and did a fresh installation of High Sierra on it. This didn’t speed anything up, and running a disk speed benchmark tool showed that the SSD was giving a throughput of abut 30meg/second, compared to about 150 for the internal hard drive. So I did a clean install of High Sierra to the internal drive, ran the same benchmark which showed the internal hard drive running at about 30 meg/second, and the external drive running significantly quicker.

The he tests are run after the install has finished and the machine has had a couple of days to finish syncing the iCloud Drive, photos etc. Nothing significant is running in the background, and nothing shows up on the task manager as causing significant throughout on the drives.

Any my suggestions please?