As mentioned above, you might want to have a look at a few of the different forum discussions on this topic, but I'll give you a few pointers here as well.
1.) Implement a keepalive service to prevent the long first page load, sites such as host-tracker.com and pingdom.com offer great services
2.) Ensure that you have proper caching settings configured for dotnetnuke
3.) If you are still concerned, potentially look at something like PageBlaster from Snapsis.