phyber: TL;DR - Try using a VPN if you have one! Worked for me (using PIA).
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To anyone having issues with terrible download speeds, try connecting to a VPN in a nearby country.
I live in Perth, Australia and was getting about 10KB/s - so roughly 100 seconds to download one MB. This was using browser download, not Galaxy client.
I tried Galaxy client and same issue - it actually timed out and said server error.
I happen to have a subscription to PIA - a VPN service. I connected mine (to overseas Singapore server) and tried again...
BOOM - downloading at 1 to 1.3 megabytes per second.
Who can say why? Could be ISPs intentionally or accidentally killing speeds to GOG servers. Hard to say.
Whatever the reason, if you are having these issues try signing up to a VPN service or borrowing a friend's. PIA is pretty cheap, about $30 a year IIRC, and this is not the first time I've noticed a particular site loading faster via an overseas VPN than "direct" from my ISP.
Tried this suggestion, but VPN made no discernible difference > after 24 hours the d/l sppeed has seen a max of .6 MB/s, but mainly .4 MB/S.
I have now been downloading witcher 3 for 3days and only 25GB of 34GB is complete. Absolutrely ridiculous!