Let us take a this hypothetical situation. You have to serve a web page. You want the whole page to be sent back in 500 ms (milliseconds). If your user has a good network and he is not too far from your webserver, you can further assume that around 50 ms will be spent on the network. This means that you have 450 ms to collect all the data about this web request, do the fancy manipulations (sorting/filtering/updating files etc.) and serve it to the user. You need to make four external calls to get this data - 2 of them to an external web service and 2 of them to your own database.
Now assume that one of your external webservice calls take one second to send back the result 50% of the time and one of your database queries can take upto a second to give back the result 25% of the time. What will you do to make sure none of your users ever have to wait for more than 500 ms to get back the page? (500 ms excludes the time taken to download the images/css/do fancy javascript magic).