the only thing I can tell you is that I have a 6MBit DSL line it works great. I've even tested then WAN port for higher speeds (cause I'll probably upgrade to 16MBit soon), and it seems capable of delivering 20MBit.
Maybe you have played with the bandwith management? My suggestion would be ... do a factory reset, flash Olegs latest firmware, factory reset again and set up only the most necessary things for now. Then test.