The 900,000 items import failed (I have not investigated the cause).
Importing 100,000 more items in foreground mode did work, but:
1.- Immediately afterwards, I received the following warning (there are no images, files or media in the current 200,000 items, only title, description and 9 url fields):
/home/plone/52/buildout-cache/eggs/ZODB-5.6.0-py2.7.egg/ZODB/Connection.py:580: UserWarning: The <class 'Products.CMFPlone.Portal.PloneSite'> object you're saving is large. (19859973 bytes.)
Perhaps you're storing media which should be stored in blobs.
Perhaps you're using a non-scalable data structure, such as a PersistentMapping or PersistentList.
Perhaps you're storing data in objects that aren't persistent at all. In cases like that, the data is stored in the record of the containing persistent object.
In any case, storing records this big is probably a bad idea.
If you insist and want to get rid of this warning, use the large_record_size option of the ZODB.DB constructor (or the large-record-size option in a configuration file) to specify a larger size.
warnings.warn(large_object_message % (obj.class, len(p)))
Do I need to change something?
2.- Now, deleting a user takes 14 minutes (very uncomfortable and inefficient ...).
Does anyone know why this happens and if it could be solved?
I'll keep testing ...