If some one is interested:
Miscellaneous -> "Max. simultaneous conversion/new Download/plugin jobs" was set to 1.
(Added 822 books in 2635.6 seconds)
Now I have 5 books failed getting file name and a "BusyError: database is locked".
The time to add a book was 2.8 seconds and higher if succeed and 2.3 to 2.6 seconds, if failed because the database is locked.
Failed:
<2.3 (0x)
2.3 (2x)
2.4 (1x)
2.5 (1x)
2.6 (1x)
Success:
2.7 (0x)
2.8 (1x)
2.9 (23x)
>= 3.0 (793x)
debuglog.7z.zip