All you need to do is run the Speed Simulator, select a speed that you want to simulate and change the proxy in your browser to localhost and port 8080. In Firefox you do that in Tools > Options > Advanced > Network and the button Settings. Just hit start in Speed Simulator afterwards and use the browser to navigate to your website. You might be surprised about the time that some users have to wait before they can see it at all.

The difference to websites that do check the loading time of your site simulating various speeds is that you are experiencing the speeds by yourself. 20 seconds does not seem much but if you experience this by yourself you might feel that it is to much.