Publius: I thought fastpath doesn't work with pppoe and/or NAT?
No to pppoe but yes to NAT

The whole idea is to speed up NAT transactions, see http://download2.mikrotik.com/news/news_65.pdf
Mikrotik are quoting up to 900Mbps NAT on an RB2011 WAN port.
See http://wiki.mikrotik.com/wiki/Manual:Fast_Path for supported interface type and conditions. Note ChangeLog for 6.30rc19 has additional new features now.