Hmm IIRC adding doesn't involve any operations that scale non-linearly with database size. If you have the source, adding a few
Code:
from time import time
print time.time()
in the _add_books function in gui2/main.py should show you where the slowdown is occurring (you will have to run the GUI using
Code:
calibre-debug -c "from calibre.gui2.main import main; main()"
to see the output