Please, see the documentation:
https://winscp.net/eng/docs/ui_transfer_custom#calculate:

The option cannot be disabled for background transfers, when using multiple connections. Though for background transfers with multiple connections, the calculation does not add time to a transfer, as directories are not listed again for the transfer itself.

Also https://winscp.net/tracker/1691

Hi,
Version 5.17.3 (10325) seems to have reverted to a prior behavior where unchecking the "Calculate total size" checkbox in Options > Preferences > Transfer Settings is not having any effect. I have unchecked this box in all Transfer Settings Presets, yet the Queue clearly shows that Progress = Calculating before any files will begin transferring. This essentially doubles the time to do a transfer, or worse it seems that the Calculating phase takes longer than the transfer itself.

I seem to remember this was fixed in a previous build?

Thanks