http://arav.i2p/stuff/article/hardening_mikrotik
dstnat in-interface-list=WAN FastTrack mechanism is described in documentation . But in short, fasttrack mechanism sends traffic via short path past the CPU, in other words, it offloads traffic to a switch chip. While it drastically reduce load on a CPU, we lose ability to work with traffic allowed by this rule, like packets marking, and other things in a mangle section.