During the first search, the dictionary is downloaded once and for all in your browser. It means that, since data are already in your computer, you can work offline.
Search is extremely fast. The dictionary is in your computer, thus, there is no need to send a query to a server and wait for its answer.
To search a word based on it pinyin, you do not have to know the tones. For instance, looking up 'gonggongqiche' will return : '公共汽车 (bus)'.