Download process is VERy long

Hello,

On an intranet I am using your plugin.

each user has its own page where he can download documentation I integrated there with shortcodes.
But this process takes really long : you click on the download button and then nothing happens for close to one minute.Most users have left before the download window opens…

Any idea what I should check ? i have no error whatsoever in debug mode, hence no idea why it takes so long (and this has nothing to with file size).

thanks for your help