As also others are having similar problems, for example:
http://forum.mikrotik.com/t/rb2011-slow-internet-even-with-fasttrack/123079/1
http://forum.mikrotik.com/t/rb2011-slow-wireless-speeds/137594/1
http://forum.mikrotik.com/t/super-slow-wifi-speed-on-default-configuration/137075/1
http://forum.mikrotik.com/t/rb2011uias-looses-about-40-megabits-of-thgougpput/136942/1
then If You finish Your experiment, please let us know the final result - did You get same or ever better speed, then Your old router did provide and with what config?