ISP's are tricky these days. Naysayers say complaining to your ISP may not do much. I've done some research to help myself understand recently that I might as well pass on to you.
The issue of Net Neutrality has become a political hot potato. Some feel it’s an insidious scheme on the part of the powers that be to control the Internet, whereas others say it’s a good thing, a way to keep your ISP from deciding which traffic you’ll receive, and at what level of performance.
ISPs are increasingly deploying a variety of "middleboxes" to monitor and to manipulate the performance of user applications. Most ISPs do not reveal the details of their network deployments to their customers.
What some people call "throttling" (when an ISP purposely lowers the speed of a user's internet connection), ISP's call a policy of " congestion management" or "traffic shaping". As in, if its network suffers undue congestion, it will slow bulk data transfers for network storage systems, FTP downloads, software updates and peer-to-peer transfers and torrents. Meanwhile, what they deems more time-sensitive data, such as web pages, gaming, streaming videos and instant messaging, will get priority. Moreover, often they also throttle only on certain ports.
Since Steam content is delivered (I think) using a proprietary file transfer protocol AKA FTP "shaping" could be limiting the speed of transfer for these items. Also, some "shaping" includes temporarily increasing your download speed (boosting) for the first 18 to 22 megabytes of a file when extra bandwidth is available.
You can test if your ISP is shaping your traffic with a tool that will allow users to track the network openness of ISPs like Cox and Comcast with a tool called the Glasnost test
http://broadband.mpi-sws.org/transparency/ which tests whether certain applications or traffic are being blocked or throttled on your connection. I ran all the tests (about 8 minutes a piece) and learned some surprising things!
Some people think it is unfair that ISP's are treating their protocols like second class citizens. Legit sites such as Open Office and some Linux distros that use BitTorrent to distribute files legally and Vuze and iMesh that provide Legal P2P are being given a “non-time-sensitive” designation.
One should stay on top of your ISP's pricing and plan points. My ISP recently changed/upgraded plan types but kept pricing points, affecting my download speed before I noticed and read the very fine print in my bill.
My ISP offers
Ultimate up to 55 Mbps for $89.99
Premier up to 25 Mbps for $64.99
Preferred up to 15 Mbps for $46.99
Essential up to 3 Mbps for $32.99
The FCC has concluded that advertising the "up to" speed is so inaccurate (and so confusing to consumers) that something better should be tried, recommending that a standard truth-in-labeling form should be drafted by the FCC.
Lastly, less-than-ideal speeds aren't necessarily the "fault" of the ISP, sometimes.
Crufty computers (You can only access the Internet as fast as your PC will allow you to)
Older slower routers/modems (many older units are not capable of transmitting to/from the internet faster than 10 Mbps, even though the local ports transmit in 100 Mbps. Check the manufacturer's web site for firmware downloads.)
and misconfigured WiFi all play a role.
Hope that helps some...