Upload traffic causes total web freeze except YouTube

Hi, for the past year I’ve been having persistent issues with my KPN WiFi.

Whenever I upload files to any cloud service: Google Drive, MEGA, even YouTube, every website stops working except YouTube. This never used to happen, but now it seems that once my upload traffic hits a certain point, KPN heavily throttles my connection on all sites other than YouTube.

I’ve already contacted customer service, but they won’t acknowledge the issue.
Is anyone else experiencing this, and is there a known fix?