Does anyone else have a problem with the Gog Downloader using a TON of PC resources while it's downloading? And it doesn't seem to use my entire bandwidth to download stuff. Steam uses my entire available bandwidth, why can't this? Is there a setting im not seeing?

Can't help you with the system slowdown, sorry. As for the download speed - check the downloader's settings as there is an option to limit the download (it may be on by default)

Downloading something now. Task Manager shows 36,164 kB memory and 0 - 1% CPU use as I'm downloading a 2.3 GB file. It's also using all of my available bandwidth, which is 10Mbps.

You can try messing with the "Number of concurrent connections" setting, and also the Download speed setting just below that one.

Do you use ZoneAlarm? It's known to do just this. At least when I tried it a couple years ago.
I think it was torrents that messed it up for me in this way.

I had a similar issue, but it wasn't the downloader itself that had heavy CPU usage, but the antivirus.

Not at this end, here I leave it to my ISP. Last Saturday I was getting 8bps.
Everything else on the machine works fine, it is just the internet is buggered.