Hi again.
As far as I know, Itch nowhere commits to maintaining a minimum upload or download speed. That at some point it works better for you is not a commitment that it should always be that way and 15 minutes for 300 MB is not a bad speed.
If I have to guess, these dates usually have a lot more people uploading games, due to the holidays. This is possibly the problem. Still, if you think there is a problem with the servers, you can report it or consult support. (On this forum the only person who might have information is the admin and I doubt he is constantly watching this forum)
I understand that it may bother you, could do in a few seconds now takes minutes. But there is a difference between something bothering you and it objectively being a problem. For the same reason, If your question sounds like a tantrum instead of a reasonable question, it is very possible that people will ignore it.
Objectively the uploading games is working.
In the meantime, my advice is to try use buttler, that way you can automate the upload of several files.
Good luck and I hope your upload speed improves.