timppu: Since lgogdownloader can download "Galaxy downloads" using the Akamai CDN, would it be possible to implement downloading also offline installers from Akamai, and not Fastly?
To my understanding, the Galaxy client does download the offline installers using Akamai, if you choose to download your offline installers with the Galaxy client, instead of your web browser.
Why use Akamai instead of Fastly? Because for many (all?) people Akamai downloads seem to be far faster than Fastly servers. You can see this if you try to download offline installers with your browser and the Galaxy client.
I have a 600Mbit/s (75 MByte/s) internet line, and with a browser I can download offline installers at about 18-20 MByte/s which is ok. The fastest I've seen is around 36 MByte/s but haven't seen that for awhile, and for many Fastly seems to give quite poor download speeds, like under 1 MByte/s.
However, when downloading offline installers with Galaxy, the last time I tested it I got up to 70 MByte/s download speeds, so over three times faster than what I get with Fastly.
It wasn't possible few years ago when I had lots of issues when GOG was using Highwinds CDN while I was connected to my usual VPN.
I even asked from some GOG people if it was possible to choose CDN for offline installer downloads and they said it wasn't possible. That could have changed since then.
However I just installed Windows in a VM and snooped the Galaxy traffic using mitmproxy when downloading offline installers with Galaxy and at least for everything I tried it only connected to Fastly CDN.
I'm guessing the faster download speeds you are getting with Galaxy are because it uses multiple threads to download a single file.
Galaxy seems to be downloading in 10MB chunks using normal http range requests on the same url that lgogdownloader gets from the API for offline installers.