The headline feature is the claimed 867 Mb/s figure for its performance on the 5 GHz frequency band, when using an 11ac router. This is listed because the theoretical maximum for a single antenna transmitting using the AC specification is 433Mb/s. The W900U contains two antennae and therefore its maximum theoretical bandwidth is 867 Mb/s (not 866Mb/s as the numbers are rounded to the nearest integer).
In the first close-range test the adapter achieved a speed of 184 Mb/s. This is in line with the speeds achieved by the current D-Link and Netgear 802.11ac USB adaptors but a little slower than the 200-209 Mb/s of the Asus adapters. At this distance the 802.11n adapter also reached exactly 184 Mb/s.
In the second test the W900U reached a speed of 149 Mb/s, compared to the 802.11n speed of 67 Mb/s. This is also faster than the 125 Mb/s of the D-link DWA-182.
In the final test the W900U returned a speed of 143 Mb/s, almost twice the 86 Mb/s we acheived from 802.11n.