Upload speed to server

To upload files to Dolibarr that is hosted on my own Debian server online, is very, very slow, but uploading files to other applicatons like Drupal and Prestashop located on the very same server is very fast. It is also very fast to upload files directly to the servers file system.

My conclusion is that it has to do with Dolibarr, but I can’t figure out what the culprit is. So why is it so slow to upload to Dolibarr, but not to other aplications on the very same server, and what can I do about it?