Help me understand. I've been trying to install this on and off for days. It will start (and top out at) around 25 Mb/s, then drop like a rock within a few minutes. What gives?

GOG uses a content distribution network (CDN) that is slow. This has been a well known issue for awhile now and there is nothing you can do to fix it.

Not universally so. I get around 50-70 MBytes/sec (= 400-560 Mbits/sec) with Galaxy (depending whether I install a game with it or download offline installers; oddly downloading offline installers seems faster for me than installing game directly with Galaxy), and around 20-34 MBytes/sec when downloading offline installers with a browser.

Also, it is different whether you are talking about using Galaxy, or downloading with a browser. GOG seems to use different CDN service providers for those two. As my numbers above tell, I also get considerably faster speeds with Galaxy, compared to the browser (but I consider the browser speeds ok too).

It is hard to tell for sure especially for Galaxy, why the download would start great but slow down to crawl, like in that case. I recall some had issues with their antivirus acting up with Galaxy and causing slow download speeds. With web browser downloads there are certainly known speed issues with many parts of the world where the download speeds are slow all the time.

Then again it depends what people report as "slow", for someone not getting over 50 MBytes/sec in Galaxy is slow (because he has a 1Gbit/s internet connection and expects to get that speed everywhere apparently), for some browser downloads that are limited to 5 Mbytes/sec per file (the speed multiplies if you download several files at the same time) is slow, and then there are cases like this where the download IS objectively slow, not a matter of opinion.

Another possible explanation, if most of the time it seems to be downloading fast but then there are times it slows down, maybe that is just how Galaxy works, ie. it does some other stuff ("installing" the game) besides downloading, which shows as slower download speeds? I've seen that behavior with other clients too, sometimes it decides to stop downloading and does something else because e.g. the hard drive activity is still high, etc.

Then again if it is slow most of the time, then that theory is probably not the correct one.

If you are repeatedly re-starting the download, it will go very fast until it reaches the point at which the download stopped last time, and then will slow down to whatever normal GOG speed is in your area.