From USB Complete:
The SuperSpeed signaling rate, or speed of the bits on the wires in each direction,
is 5 Gbps. The USB 3.0 specification refers to the rate as 5 GT/s
(GigaTransfers per second). The 8b/10b encoding increases the number of bits
to be transmitted by 25%, so 5 Gbps on the bus translates to 4 Gbps, or 500
MB/s, of unencoded data. Framing, error detecting, and other protocols reduce
the theoretical maximum data throughput to around 400 MB/s in each direction.
In other words, the transmitters and receivers support a 5-Gbps signaling rate in each direction. The amount of useful data being transferred is about 400 MB/s max.