The following was obtained from apple support. The original page can be viewed here: http://docs.info.apple.com/article.html?artnum=303192
If you are experiencing OSXvnc issues via VPN or if your broadband Internet connection (cable or DSL) goes slower than expected, your Internet Service Provider (ISP) may suggest that you change the maximum transmission unit (MTU) as a troubleshooting step.
Important: Changing the MTU is recommended only when done under the advice of your ISP or an experienced troubleshooter. Improper configuration can result in a loss of connectivity.
If your computer connects to broadband over its Ethernet port, follow the steps for Ethernet. If it connects wirelessly, follow the steps for AirPort.
Steps for Ethernet
1.) From the Apple menu, choose System Preferences.
2.) Select the Network pane.
3.) From the Show pop-up menu, choose Built-in Ethernet.
4.) Click the Ethernet tab.
5.) From the Configure pop-up menu, choose Manually (Advanced).
6.) Select the radio button for Custom.
7.) In the field to the right of Custom, enter your new setting. Some ISPs may have a specific suggested MTU value. If you’re changing it without a specific recommendation, make incremental changes such as 1500 to 1400 to 1300, and so forth.
Changes made to your Ethernet MTU settings will be persistent, meaning the new values will still be used after you restart your computer.
Steps for AirPort
Note: For these steps, you must be logged into Mac OS X as a user who has administrator privileges. You can see which users are administrators in the Accounts pane of System Preferences.
1.) Open the Terminal, which is located in the Applications folder, inside the Utilities folder.
2.) At the command prompt, type the following command. In place of “” enter the number for the MTU value you wish to set.
$ sudo ifconfig en1 mtu <value>
For example, this would set the MTU value for the AirPort interface to 1400:
$ sudo ifconfig en1 mtu 1400
3.) You will be promted to enter your password, and then the command will be completed. You can verify the setting of the MTU value with this command:
$ ifconfig en1
This will return all current information about your “en1” (AirPort) network interface.
Note: The AirPort MTU setting made via Terminal is not persistent, meaning that you will need to repeat these steps each time you restart your computer.