It's probably just the servers you are connecting to.
Have you tried using download managers that connect mulitlple times to a server? They are not always usable because of bandwidth limitations placed on the servers not allowing multiple connections.
My application of choice is called "down them all" which is a benign FireFox plugin. Again, doesn't always work but if you set it to connect 5 to 10 times and the server you are connected to allows it, you will max your speed out.
I have 20/4 FIOS and I regularly hit my cap of 2.6 to 2.8 MB/s (note that's MB, NOT Mb). Hitting almost 3 megabytes per second is pretty quick. Wasn't long ago that hard drives weren't able to sustain that kind of transfer rate.
Anyway, just a thought.
edit: Had my FIOS service level listed wrong.