If you turn your broadband router off every day then you probably never let it reach it's optimum speed. A broadband line goes through a process called "training" after being connected. The line & router will start to communicate at a fairly low speed and, if stable for long enough, will increase this in stages over a period of 2-3 days until it reaches the maximum stable speed for your line. If you turn your router off every day then this process starts again everyday and you'll never get the best out of your line.
(with thanks to LCDMAN)