Thanks both, this has been very helpful.
Hmm. I looked at your log. The Metallica tracks that are shown there appear to be adding to the library in a normal way. Well... at least as far as I can tell. But I'm no expert on reading the MC logs.
All these files are already in the library, so MC just does some 'empty' steps.
The one's with the "1" at the end will be, I'm almost certain, from the .cue files.
If most of your music has accompanying cue files, I would suggest removing the .cue file extension from your auto import rules (unless of course, you really really need them in your MC library?
This turned out to be correct - after disabling the cue files, MC's Auto Import now works like a charm - runs in seconds. Now I understand where the ~2600 is coming from - this is roughly the number of albums (i.e. .cue files) in my library.
Looks like some bugs/limitations in MC's .cue import implementation.
However...
After testing the outcome I have found a few instances, where cd's were ripped as single image files, not separate tracks. MC now does not 'see' the tracks inside. I believe there are not many such cases in my library (I have sorted everything by duration and spot-checked a few), so maybe it is not a big deal, but not ideal. Often it is hard to split such images when they are non-Red Book PCM (I have a few hi-def vinyl rips and some 24-bit editions etc.).
Do you have any suggestions on the best file splitter? I am normally using CueTools , and it does it perfectly but does not support non-Red Book PCM.