Some attempts to answer:
1: I do not know anything about tagscanner. If it uses an internal database - that might be a reason. How long, by the way, does it take iTunes to build up a library of 70000 tracks? (No, don't answer)
-
If you create a separate folder for the new tracks and read only that one, you are quicker - less data, less time.
If you undertake the effort to load the files initally, filter for the files that lacks data (and has to be tagged therefore) you could move these files with the help of the converter tag - filename or filename - filename to a new location. -
you get the filter input filed by pressing f3. there you can enter criteria as you require.
-
see the FAQs for swapping:
/t/967/1