Speed is different, because Mikrotik is sending too small payloads for every allowed transmission slot.
When the payload (blue part) is smaller, like in this extreme exemple , then the rest is the same, and the overhead % goes up a lot, and there is still the same arbitration time to be added here.
http://forum.mikrotik.com/t/help-mikrotik-capsman-wireless-download-speed-max-200mb-but-pcs-mobiles-link-speed-is-866mbps/142677/1
wifiwave2 does like other off-the-shelf drivers, with large A-MSDU(MPDU) and A-MPDU sizes. http://forum.mikrotik.com/t/wifiwave2-expected-performance/160075/1
By focusing on wifiwave2 only, Mikrotik has been throwing the baby out with the bathwater, if they do not succeed in bringing in the unique goodies of the legagcy driver.