Starting from v6.29rc9, we introduced new and existing feature – FastTrack. Easy way to make your Firewall/NAT router up to 5x faster.
*) ipv4 fasttrack fastpath – accelerates connection tracking and nat for marked connections (more than 5x performance improvement compared to regular slow path conntrack/nat) – currently limited to TCP/UDP only;
*) added ~fasttrack-connection~ firewall action in filter/mangle tables for marking connections as fasttrack;
*) added fastpath support for bridge interfaces – packets received and transmitted on bridge interface can go fastpath (previously only bridge forwarded packets could go fastpath);
*) packets now can go half-fastpath – if input interface supports fastpath and packet gets forwarded in fastpath but output interface does not support fastpath or has interface queue other than only-hw-queue packet gets converted to slow path only at the dst interface transmit time;
*) trafflow: add natted addrs/ports to ipv4 flow info.
http://www.mikrotik.com/download/share/FastTrack.pdf
http://forum.mikrotik.com/viewtopic.php?t=96302