Hello,
As you said, there are a lot of users with the same issue if you use the search engine, so I guess that's not working for all users. At the moment, I see a lot of thread asking for help and no answer on this question from itch.
The issue does not come from my provider or the 3 other different providers I tried. I am working directly with IT, so when I say I tried it from work, I can assure you that I checked with Janine first (and she is not grumpy, how dare you, she just don't like people ;) .
I asked 2 of my friends living in switzerland with differents providers and they have the same issue : really slow download, other websites are working fine.
Yesterday, I could download a 250mo game. Took me all day. On humble bundle or steam, download is full speed (of course, I don't download anything else while using Itch).
I guess I can forget the +150 games bought on itch and use other platforms now.