"The operation has timed-out" NuGet warnings in TFS2010 builds

I have a solution (configured for "package restore") whose projects reference packages both from nuget.org and our own repo. You can see the nuget.org URL and our repo's UNC path in the log snippet below.

Whenever I run a build on our TFS server, I see warnings like this in the log (company names ****'d out):-

     3>RestorePackages:
         "C:\Builds\1\*******\.nuget\nuget.exe" install "C:\Builds\1\******\packages.config" -source "https://nuget.org/api/v2/;\\\\OUR_TFS_SERVER_NAME\\NuGet Repository\\"  -RequireConsent -solutionDir"C:\Builds\1\******"
         Restoring NuGet packages...
         To prevent NuGet from downloading packages during build, open the Visual Studio Options dialog, click on the Package Manager node and uncheck 'Allow NuGet to download missing packages'.
         Installing 'CommonServiceLocator 1.3'.
         Installing 'Castle.Windsor 3.3.0'.
         Successfully installed 'CommonServiceLocator 1.3'.
         Successfully installed 'Castle.Windsor 3.3.0'.
     2>EXEC : warning : The operation has timed out [C:\Builds\1\******.csproj]
     2>EXEC : warning : The operation has timed out [C:\Builds\1\******.csproj]
         Installing '***.Core 1.1.1131.1'.
         Installing '***.Nice.Common 1.0.1241.2'.
         Successfully installed '***.Nice.Common 1.0.1241.2'.
         Successfully installed '***.Core 1.1.1131.1'.

And an example build warning from a subsequent project in the solution:-

     3>EXEC : warning : The operation has timed out [C:\Builds\1\******.csproj]
         '***.Core 1.1.1131.1' already installed.
         All packages listed in packages.config are already installed.

The solution *does* always build successfully, however there can be a dozen of these warnings, and each timeout appears to last 30 seconds, so it's really affecting build times. It's been an intermittent problem for months, but has been gradually getting worse and is now fairly consistent in the number of warnings seen, and I don't know why.

I did wonder if it had something to do with the number of .nupkg files in our repo, as I don't clean up older versions. There are currently approx 100 files (20 versions of each of our 5 packages), which isn't exactly a lot! And it doesn't explain why the package still ends up being installed despite the timeout.