With steam my download speed works at its maximum capacity of 8.6 mb / s, with the same ORIGIN works above 8 mb / s.

But with GOG the speed fluctuates from 2.0 to 2.9 mb / s and sporadically increases to 4 mb at most.

What is failing them?

Galaxy, browser, or Downloader?

Galaxy.

Try your browser and/or the downloader to see if it makes a difference. I use the DLer and the speeds are very good. The odd time I've used Galaxy for DLing and the times I've used it for updating, it's been lightning fast as well.

The reason is the heat here in Europe. Because of it the data expanded and doesn't fit any more in the cables...