Good question and one I've been wondering about. I'll tell ya something strange; is that the 865 versions with
out CSA
are faster than the 875 counterparts
with CSA! Examples....IS7/IS7-G is much faster than the IC7/G; Asus P4P800 is faster than the P4C800, etc. It can't be because CSA slows a mobo down, that just doesn't make since. It's probably because the PAT mimicking of the 865 is better than the true PAT of the 875 versions.....just a guess.
In some anandtech CSA article they mentioned the CSA was much faster than a non CSA
on their in-house gigabit LAN if that's any help to you.
Link.
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BTW if you think I'm lying Peter, here (removed) is a screenshot from a few minutes ago. Although only 456KB at this time, it's usually over 500KB overnight. (Link will remain active for only a day or so).