A burst of speed to gain position is what is needed for most shots to succeed. It is imperative to be in the most fantastic position to hit every ball if you want to hit a great shot. Most of the time, the only way to gain that position is with a burst of speed.
Where does that burst come from? You! If you have it in you! It takes work, hard work and a lot of it to develop the kind of speed & strength that will carry you to the next level.
Think about Federer, Djokovic, Serena and Azarenka and how hard they are working for maximum positioning on every shot.
Want to play like them? You have to have "bursts of speed" like them too!