The graph shown in this slide assumes that the typical parallel flash access time is 90nS with sequential reads. This equates to an 11 Mbps through-put for 8-bit parallel Flash and 22 Mbps for 16-bit parallel Flash. Typically there is no buffering in parallel NOR Flash, so the data rate is constant as the number of bytes per fetch increases. However, serial Flash supports buffering, also known as bursting, so it can buffer significant amounts of data. This buffering allows Quad SPI Flash to achieve through-put data rates of 40 Mbps at 80 MHz,  which is equivalent to running two 16-bit parallel Flash chips at the same time.