Very slow performance after installing RAID controllers

[David Brodbeck]

Maybe you already checked this, but a duplex mismatch can cause sporadic
network slowness.  (i.e., a card that's running at half duplex while
connected to a switch that's full duplex, or vice versa.)  It'll work fine
if you ping the machine, or anything small like that, but as soon as you try
to move a lot of data everything goes to pot.  Amazingly enough the
collisions don't always get logged by the OS of the machine at fault when
this happens.