I answer to myself in case anyone else was having the same issue. No, it is not normal to have such a RAM usage merely for adding users in the database. The issue was with a (poorly developed) prepackaged plugin of the theme Listingo, which was constitutively querying the user database in each page, thus slowing down everything. Adding the users to the database does not increase RAM consumption now, only -obv- when the db is queried.
