I installed Media Monkey 4.1.13.1801 on a Windows 10 PC with an SSD.

I attempted to import my songs but it would always stall at around 12,000 songs.

I was convinced it was not because of a bad file, because I could move the directory with the failing file out of the way, and the import would fail on another file, but still at about 12,000 songs.

Also, there were occasionally errors saying something like:

Error executing SQL statement "INSERT INTO Covers ...": database is locked (5,5)

If I killed Media Monkey I noticed that a Media Monkey background process was still running, and that the MM.DB file was increasing in size!

I took a look at the DB using a Sqlite browser and saw that although there were only 12,000 rows in the Songs table, the Covers table had 97,000!

Running this query:
----
select IDSong, Count(*) as cnt
FROM Covers
GROUP BY IDSong
HAVING Count(*) > 1
----

... showed that a bunch of songs had 215 covers, which of course is ridiculous!

I deleted the library (File -> Clear Database), then went into Tools -> Options and made some changes:

Library: unchecked "Scan file directories for Artwork"

Library / Tags & Playlists / Artwork: Changed "Image location" to "Save image to file folder" (Don't know if this step was needed)

I then reimported and all 41,000 files were imported with no problem! The import also seemed to go faster!