I have a problem with very slow network access under very particular but seemingly unrelated conditions.
Using Debian "Squeze" (debian6-19-04-2012):
When running LXDE with keyboard and mouse connected via a self-powered USB 1.0 hub, network access is slow. If I ping my router ping times are all over the place ranging from 1ms to several tens of seconds. On average around 5 seconds but I have seen responses up to 60 seconds.
Outside of LXDE - just plain CLI, the ping times return to normal.
With the keyboard and mouse connected directly to the RPi even in LXDE the ping times return to normal.
I have tried this powered with an after-market Blackberry PSU and from a PC USB port.
I looked in /var/log/messages, and there were many errors like:
smsc95xx 1-1.1:1.0: eth0: Failed to read register index 0x00000114
smsc95xx 1-1.1:1.0: eth0: Failed to read register index 0x00000118
reported ever few seconds. I was hoping to post a log, but the flaky network connection made that impossible. Now I have found that it is stable without the USB hub, I may try again if the information might prove useful.