Disable VS' "downloading public symbols"

You can do this from the Symbols option page

• Tools → Options
• Go To Debugging → Symbols
• Uncheck all of the listed symbol file locations

Also you may want to disable "Enable .net framework source stepping" in Tools → Options → Debugging settings.

From VS Main Menu -> Debug -> Options and Settings
Make sure all the following parameters are configured as follows:

1. Debugging -> General -> 'Enable Just My Code' is Enabled

2. Debugging -> General -> 'Enable .NET Framework source stepping' is Disabled (this might occur automatically after setting step 1)

3. Debugging -> Symbols -> All symbol files locations are unchecked

Disable "Enable .net framework source stepping" in Tools → Options → Debugging settings.