Iperf udp test show only 50 % of bandwidth

Check if your switch (your network cards should already support it) is accidentally not set to auto-mdix or if there is fixed value and you got a mismatch. This can cause the problems you describe, I've seen the exact same behaviour in our own network with a mismatch. Sometimes auto-mdix might not work and you should also try setting it to the same value on both sides manually.

http://en.wikipedia.org/wiki/Mdix#MDIX

When tcpdump is running, it will be fairly prompt at reading in the incoming frames. My hypothesis is that the NIC's packet ring buffer settings may be a bit on the small size; when tcpdump is running it is getting emptied in a more timely manner.

If you're a Red Hat subscriber, then this support article is very useful Overview of Packet Reception. It has some things in there that I don't think you've considered yet.

Consider how your system is dealing with IRQs; consider increasing the 'dev_weight' of the network interface (meaning more packets read from NIC to user-space); look at how often the application reads the socket (can it use a dedicated thread, are there known issues/workaround regarding scalability).

Increase NIC frame buffer (using the ethtool command -- look at the --set-ring etc. arguments).

Look at 'receive side scaling' and use at least that many receive threads to read in the traffic.

I wonder if tcpdump is doing something cool such as using the kernel support for packet ring buffers. That would help to explain the behaviour you are seeing.