Hi Cornel,
One possible cause that the data upload might be more expensive the second time around is if you had created indexes after the initial upload. If you have a large amount of data and your application is not yet being used, it is often best to upload the data, then create indexes. The index creation time might be longer, but you'll save on API CPU cost. This is just one possibility, I think I'd need to know more about your application.
Thank you,
Jeff