If you are cooking solid foods such as a roast, potatoes, etc., you can speed up the initial cooking process by putting the items in boiling water for a few minutes while you crock pot warms up.   This will bring the temperature of the food up instead of forcing your crock pot to heat it up.

This also works well if you are not sure your vegetables will have enough time to cook thoroughly, you can push them on their way.

Also, if you are planning to add a bit of water to your dish, you can add boiling water to help bring the dish up to temperature.