The problem is most likely either your ISP throttling downloads, or Microsoft's download network in your geographical area.
I personally have 28Mbps internet, can download most anything blazingly fast, but anything from MS marketplace for the past year or so goes at a snails pace. Right now I'm waiting for a 160MB map pack for ME3 to download, it's been 20 minutes already and it's only at 70%.
I suspect the problem is with the MS content network, as I have had this problem in my location on different ISPs (DSL and cable) for over a year, but before that it worked quite fast on the same ISPs.
There's not much you can do about it, their support people are all drones and will most likely only offer apologies and little else.
For what it's worth, I can download their streaming media fast enough to stream HD video within seconds of starting - that's most likely content delivered by another provider than the one that does the xbox live downloads though.