The fastest SD cards are 300 megabytes/second, not megabit. All my Sandisk Extreme Pro cards, for example. I also have a bunch of Extreme Pro CF cards that are 160MB/s.
And then there's CFast - speeds in the 400MB/s range.
Apologies, I messed up my capitalization. That ought to have been "MB". You're right that things like compact flash are faster, but USB 3 has can handle over 600 MB per second.
Still not sure I understand what you mean. If you have a slower USB, slower card reader, or slower card, all components are bound by that speed. The USB 3 supports a much faster than the card speed; I'm guessing most readers are similar speed. The cards themselves are the limiting factor here.
And then there's CFast - speeds in the 400MB/s range.