Since we have a lot of users who downloaded the game already, few things I'd suggest checking
- see if decreasing number of download threads helps (it's in the app settings). You may have to wait up to 10 minutes to see result
- check if temporarily quitting your antivirus or firewall helps (or whitelisting *.edgecastcdn.net)

It might also be the case that the file simple didn't propagate well enough in your region yet. For a file this big it takes a moment. Feel free to run ping and tracert to cdn.gog.com and post results here. We will know which edge server are you likely getting the file from and investigate further.

I've seen your post and rest assured we're contacting selected users and asking for the logs to triple-check what issues they might be having. It's not a global issue with the Client however - I've just finished installing the game @ 20 MB/s using it and we see lots of users getting some really high download rates. But we will look into this anyway.

Having that said I'd also like to point out two important things before drawing any conclusions
- at this moment there are times more users in the Galaxy than on the website. Any by more I do mean factor of tens
- Galaxy installation mode downloads completely different files in a completely different way than standalone installer from the website / Downloader / Client. It also means completely different propagation for these files
- we're currently pushing dozens of petabytes per hour (and yes vast majority of that goes through the Client)