[ixp1200] ix bus throughput limitations

Hi, I have carried out some testing on a single thread process receiving and counting packets (code from chapter 5 from ixp 1200 book) When it runs at wire speed at 177Mhz and Ixbus at 66 Mhz it has a receive rate of 350 Mbps at the IX bus. The same rate is achieved when running at 200Mhz, Ixbus at 66Mhz. I would expect the receive rate to be faster. Does anyone have any idea as to why the receive rate isn't higher for 200MHZ? Thanks Liam Noonan Univeristy Of Limerick