Opening a new browser tab (or window) is not the same as opening a new connection. Browsers are really good at re-using already open connections and the 20 seconds delay is evidence of that. You can verify this by printing out the remote IP + port in your server, it will be the same for both requests.
In Firefox you can force a new connection by pressing ctrl+F5, Chrome probably has a similar feature. But you could also write a little (multi-threaded) client program that does the get-request: it is not that difficult to write and will come in handy when you need to test/debug other features of your server.