i.e., the speed of your network within your house is much greater than th elink from your house to the internet, therefore your use of 3G or even 4G outside the house will look like crap because it is probably limited to 300-350 kilobits even if you have a 0.500 megabit upload (according to your ISP.
To get HD over 3G/4G your upload speed needs to be more like 2-3 megabits.
My ISP provides 1.5 mb download (incoming) but my upload is 0.500 mb and my Sling AV reports upload speeds around the 0.350 mb (350 kb) range, rarely it may hit 400 kb.
Inside my house on the wireless G network the sling stream can run 3-4,000 kbps which gives me a very good picture full screen on my laptop, wired connection is nearly twice that fast internally.
You'll find that upload speeds fast enough to carry HD even to an iPhone are not cheap, by any means.
The weakest link in the chain is your network upload speed, normal internet traffic uses very little upload data, most of it is download.
Sling is the exception.
All the hype about using sling to view TV ANYWHERE is heavily dependent on your upload connection speed and the quality of the node your iPhone is connecting through.
We have areas outside town where 3G drops to 2G then 1.5G - even data on a smart phone fails at 1.5G.