I have a similar issue.
Windows 7 enterprise x64, lenovo x250 laptops. Driver version is 12.12.80.19. Standard install of OS and used the lenovo system update to install all the drivers.
After the laptop sits for a while the network becomes very slow. To test I setup a continuous ping to a server on the same subnet. Here is where my issue diverges a bit from the thread. It's random when it happens and doesn't necessary happen when coming out of sleep or hibernation. This is happening on all the laptops that we have tested so far. Our power plan doesn't put anything to sleep other than the screen. I turned off power management on the NIC. The pings will start to drop and for the pings that make it the delay is very long, 1000ms+. If I make any change on the NIC that requires the NIC to refresh everything goes back to normal. If I unplug the cable and plug it back in everything goes back to normal.
I hope to resolve this soon as I'm rolling this laptop out to everybody in the company.
Any ideas?
Thanks
Eric