Hi, we have more than 40000 records in hr.attendance and more in hr.timesheet. But odoo gets extremely slow when the records in the csv goes beyond about 200-300. I always split the data in about 250 records. Our product database was about 50000 records and it ws meaningless to split it, so I created a module and imported the records as a data file, quite fast.

Another thing is that csv import commits the data to the db after loading all of them, so the import would get discarded quite easily if it is large enough.