Horrible Download Speed - HELP PLEASE

I know that. Apparently you don't, as OP stated mbps, all lowercase, and it looks like he should be having 1.375 MB/s according to speedtest, which by default shows Mb/s (11 in his case). That would also make his 500 - 800 KB/s logical - probably someone else in the house is using the remaining bandwidth, or other programs, or perhaps he's part of some botnet / has a trojan of some kind.

You said "just because it says 11MBps, doesn't mean you can get that download rate, it just means that is the total speed if you were multi-downloading files would be 11MBPS". "MBps".

No it doesn't sound right. In my case, Steam uses most of my internet connection (eg. over 95% of it).

Oh btw, I have worked on backbone routers on fiber lines, as well as ADSL2+, and have given support on both DOCSIS 1 and 2 lines. I've worked on all manner of Cisco hardware, including 2000 port Juniper routers (with about 10 virtual ports per port), part of which was managing the top 2000 business customers of KPN, the main Dutch ISP. Lines like Schiphol airports' dual 20 GB/s lines. Or the fire departments' national grid that had a max allowed downtime of 15 minutes, regardless of the cause of downtime, due to their SLA.

What you're talking about with "you won't get your max speed most of the time" refers to transport overhead, which is a negligibly small amount for the TCP/IP protocol (error correction bits) + in the case of DSL, due to how frames are built up (when dealing with PPPoA, which most providers use), you always have some extra transport overhead that is somewhat significant (slightly less so on DOCSIS 2 lines) along with DSL having its' own error correction, as well. The last two add up to usually take up about 10% of your line speed, excepting very fast lines. That still leaves a healthy 85 - 90% of your theoretically attainable line speed. All this "you won't get your max speed" stuff is, however, entirely irrelevant, as OP posted his speedtest.net results, which obviously already includes all this overhead.

Steam has always maxed out and used all of peoples' bandwidth - it was an issue many bitched about, due to a) no one else in their house being able to use the 'net while someone was downloading / updating a game, and b) themselves having an all but non-functioning 'net connection on their own PC. That's why now, Steam has a download limiter in the settings, under the "Downloads" tab. That's an entirely new feature that only recently was added. Before that, again; Steam used to max out people's connections.

The reason your connection doesn't get maxed out can be down to two things - you've got high speed fiber, you say? 100 Mb/s? Yeah, most ISPs offering such lines will state in their fine print that they offer speeds "up to" 100 Mb/s. No hard guarantee of that speed, ever. And usually you won't get it, either, due to local congestion. If you would, you'd be paying roughly 5 to 10 times what you're paying now for that line, and more likely have a business line with a SLA attached rather than a private line with no SLA or any guarantee of speed. It's because ISPs want to cheap out on infrastructure and hardware.

So, there's that. Then there's the fact that Steam, if its' collocation's (whatever location you've picked from the list in the "Downloads" tab) busy already with some uploads to people, they only have so much bandwidth to spare, themselves. Maybe they even have a hard limiter set on the port on their end to prevent someone with a huge fat pipe swallowing up a lot of their bandwidth and causing congestion issues and time-outs for others. With a 100 Mb/s line, in the few times you might actually be receiving that 100 Mb/s from your ISP, you'd be receiving 12.5 MB/s downstream, which is quite blazingly fast. Now let's say one of their collocations has a 20 or 40 Gb/s pipe, or two of them. Let's say, 2 20 Gb/s pipes. That's 2 x 20 = 40 Gb/s / 8 = 5 GB/s x 1024 = 5120 MB/s bandwidth. Your 12.5 MB/s line would only fit in there 409.6 times. So, if Steam were to allow you to download at that speed, they could only cater to 409.6 people with speeds like yours. To prevent too much congestion, it would be feasible to throttle some of that to keep steady connections across your hardware, and manageable speeds for all.

Is that enough info, now?