Voltage droop is a real issue for many cables not just USB ones. It's a much bigger issue when you use them for power, it's a pretty well known issue that cable length and gauge can have (a pretty big) effect on charge speeds. It's even a bigger issue on the higher end devices that will limit the charge not only based on voltage but also on current, basically if the cable can't deliver the the 2 amps they'll drop down to 500ma or even lower.

I've switched most of my USB charging cables to those which I know are 20AWG, the funny thing is that these cables aren't even more expensive I got mine from http://www.portablepowersupplies.co.uk/ and they are cheaper than most cables on Amazon....