Problem resolved
Dear ChrisA,
Thanks for your interest and advice.
I tried a different CD (19 tracks, 60 min) and disconnected from the internet: this took 12:51 and iTunes showed about 8 x all the way through and Activity Monitor 15-20% - much better.
I then ripped the same CD with internet on, to access GraceNote, and this showed about 5 - 9 x on iTunes, and about 15% on Activity Monitor and took 14:56 - again most acceptable.
So I think that this was a combination of a problematic CD and the impact of GraceNote rather than reflecting a hardware fault which was what was worrying me.
Thanks again, John