This may be something you cannot control. It is often dependent on the server on which your page resides. However, there are some issues that can be controlled. Take the example of generating an order number, if this takes to long the user may think that the page is not working. The user may then cause duplicate orders to be made or leave the page entirely.
Many web sites use a database.
If you are using a database you will need to do some performance testing in that area as well. Identify areas where the page reacts to users and check how long it takes them to react. Decide on an acceptable reaction time can be a little different in this case. You may want to decide on an acceptable reaction time for each service. Fail Generating a new account and adding an item to the cart should not take the same amount of time.