This issue affected several people after upgrading to Mountain Lion. For many, reducing their MTU size solved the issue:
- Open System Preferences.
- Select Network.
- Select your connection and choose Advanced...
- Select Hardware.
- Change Configuration to Manually
- Reduce the MTU size to 1300. Most people had good experiences with this value.
- Observe how
ping
behaves over time. You can further reduce the MTU size in steps of 4.
However, there seems to be another issue: Proxies. The solution suggested here is:
- Open System Preferences.
- Select Network.
- Select your connection and choose Advanced...
- Select Proxies.
- Turn off Auto Discovery Proxies.
- Apply the settings.
But this should be fixed with an upgrade to OS X 10.8.1 (not sure if an upgrade to Mountain Lion directly brings you to the latest upgrade, or to 10.8.0).
Actually, I would first try the upgrade to 10.8.1. If this does not work, try the proxy solution. If this does not work, try the MTU size reduction.