I don't know if you know what I'm going to tell you, but surely someone had already noticed: Apple uses USB hubs inside their laptops for things like Bluetooth or the card reader, so if we are fans of the speed of writing and reading we must take it into account.
In a recent study on MacBook Pros it was discovered that the port that is located internally with the card reader, the Bluetooth, the keyboard and the trackpad is slower than its counterpart, which in turn is connected together with the iSight and the infrared port.
This which may seem silly makes it important when the write / read speed is clearly higher on the other port, and for things like Time Machine it shows.
By the way, the fast port is the one closest to the MiniDisplay Port.
Source | AppleWeblog