I've been downloading a six gigabyte file most of the day - why would it max at 150 kbps when my Speedtest.net demonstrates that I can max over 8 mbps?

Maybe the server you're downloading from can't serve you faster?

That's the obvious answer. It's GamersGate....usually I get around 3 mbps with them.

They might have had a lot of traffic all day.

GamersGate is mostly OK, that's true, but I recall having really low speed downloads from them. Maybe they're indeed experiencing too much traffic, but it could also be a server failure/maintenance.
When your file is finally downloaded, you should try downloading from somewhere else to see if you get better results there, and thus to see if the problem was with GG. I guess it'll be your normal speed with other sites if SpeedTest told you your connection was 8MB/s.

oh, wait a second, I'm waaay off 300 KBPS per second I meant.

But still I'm dropping down to about 33 kbps

Internet gremlins.

Yes, Advanced System Care and Gamebooster has temporarily turned those background gremlins off and now I'm maxing about 450 kbps. =D