Well, it took several hours, but I got an answer back from Kovid Goyal himself (the primary author of Calibre) and he said:

And the way calibre works is
that when the library is first loaded, all the db contents are read into
memory. Searches then take place in memory with no disk activity.

The only exception to that is that you have defined a custom column that
shows what formats are available for a book. This will cause disk
activity when searching since calibre has to verify the individual
format files exist. This info is then cached.

You can work around that in two ways:

1) Remove the formats column and replace it with a approximate_formats
column that does not verify on disk status, use the template
{:'approximate_formats()'} instead of {formats}

2) Go to preferences->searching and restrict the list of columns
searched for unprefixed searches.


I went for option #2, and that solved the problem.

tanstaafl.