My 2010 Mariner handles large USB sticks the way it should. When you first plug in the stick, it may take a long time to scan it, but when you turn off the car, it knows that nothing has changed on that stick and starts up right away. If you add more songs to the stick, it will have to scan it...
USB 1 speeds would work fine for playing audio and USB 3 is backward compatible. No need to wait for the iPhone 6, except possibly for compatibility testing, but I don't think Mazda does much testing.
The reasons it takes so long to discover what's on the USB device are:
1) It's probably a...
This is almost exactly the line I got (except for the bold text) when I took our new CX-5 in for the first oil change and complained about the car forgetting where is was and always starting at the first song. Has this problem really existed since Feb. 2013 and they haven't fixed it yet?