If your router is working properly, then there is no reason for your MacBook to be slow. Assuming MacOS X 10.5:
- Launch System Preferences.
- Open your Network preferences pane.
- Click Airport in the sidebar. Note the Network Name: popup menu. Is this your router? If it is not, then choose your router from the popup menu.
- Click the Advanced... button.
- Click the TCP/IP tab.
- Set Configure IPv4: to Using DHCP. Set Configure IPv6: to Automatically. Some systems have trouble with IPv6. If this is the case, then it is OK to set it to Off.