1/ The idea is to read a CSV file, and create an item with its properties for each line. So I have to loop over thousands of lines and create an item and multiple claims associated, and it takes quite some time to do so. (atleast 1 hour to create 1000 items) I guess it's because for each line, I create a new entity and new claims, which means multiple requests for each line.

There is an API call, wbeditentity [1] that allows preparing an item with multiple claims which are written to WB in one call. its Are you aware of wikibase universal bot [2] and wikibase-tools [3]? Both cover functionality that should allow to do what you describe above and both use the wbeditentity call.