I read in some reviews on IP35-Pro that there are some issues with the Ethernet performance of this board.
 
How bad is it?

it's ~650-700Mb/s
That's way faster than domestic broadband & faster than a single 7,200rpm HDD can cope with.

Do you have the link?
I would doubt the reviewers methods or understanding of the results.
 
You would be hard pressed to show any Gig Ethernet NIC have relevent issues on a workstation.  I could POSSIBLY see it if testing the device as an enterprise server or web host handling a very large heavily trafficed site.
 
But for any type of home PC, even that of a hard hard hard-core game, I would find that tough to fathom.

I believe that this was extremely minor issue was with the rev1. The newer rev1.1 do not.

the 1.1 has exactly the same network setup  as the 1.0 - but it's still not a problem for 99% of people.

Here is the link I think he was referring to.
 
http://www.techspot.com/article/61-i...board-roundup/
 
If you look at the benchmark results for the HDD/LAN Speed you will find that the IP35Pro is capped at ~50MB/s vs 65-71MB/s for the other boards reviewed.  This is due to the fact the ABIT board uses the PCI bus for its LAN controllers and not the PCIe bus.
 
That being said I have to agree that it will be unnoticeable on a home based PC.  You will not get anywhere near using that kind of bandwidth.

the fact that the other boards are getting nowhere near gigabit speeds either leads me to believe that they either had very high overheads in their system or their test was flawed.

Sorry to bump the old one, but this is interesting.
 
If you use a few PCI cards - will this affect NIC performance?
 

they'll have to share the PCI bus bandwidth so potentially, yes.
If you consider that a problem then the IP35 & IP35-E use PCI-E LANs (the Pro uses PCI to free up a PCI-E lane to allow for keeping a PCI-E x1 socket active along with Crossfire/RAID card).