1 Gb/s is only ~100,000 packets/s at standard MTU. You literally get 10 us/packet which is a eternity. Normal fast-path router operation only really needs to consider the header of <100 bytes/packet, so you are getting ~100 ns of compute per byte of considered data and on even a 1 Ghz processor you are getting over 100 instructions per byte of considered data. Failure to achieve a measly 1 Gb/s really says more about those software implementations than it says anything about the impossibility or difficulty of the problem.