The 802.11n high throughput (HT) standard defines three modes of operation: a legacy (non-HT) mode, a greenfield (HT-only) mode, and a mixed mode where HT protection mechanisms ensure that transmissions can be detected by both old 802.11a/g/b devices and new 802.11n devices.
In mixed mode, HT protection requires that 802.11n devices send a legacy preamble, followed by an HT preamble. The legacy preamble lets 802.11a/b/g devices to avoid transmitting over HT frames sent by 802.11n devices.
These HT protection mechanisms significantly reduce an 802.11n WLAN's throughput, but they are necessary to avoid collisions between older 802.11a/b/g devices and newer 802.11n devices. If you knew that no legacy devices were present, you could configure your access point (AP) to operate in greenfield (HT-only) mode, eliminating this overhead.