I've been researching a related issue for a few months now. Unlike Steam, Microsoft Store, Google Play, or practically any software that downloads content, itch.io is allergic to packet loss, packet reordering, stuttering, or high oscillating ping. Just forget about downloading from itch.io if you suffer from any of these. In my case, I experience all of them at once :D
What I don't understand is why the allergy exists. Steam pushes up to 100mbps with my network (I clarify that my network isn't suitable for online gaming, for that I use my smartphone's LTE), but with things that use TLS or things that fail if the connection isn't stable (like flatpack or itch.io), it's impossible to download anything weighing more than 200MB.