in tiles definition, for example i created a layout with header and a dynamic body, this body can be page1 or page2. if im in page1 and i go to page2, will the header.jsp be loaded again or just the body?
Even though tiles allows you to create a composite page consiting of smaller parts, in the end, the page created is still a single HTML page. So, the answer to your question is that any time you display a different body, the header will be redisplayed also. It is after all a single page as far as the web container is concerned.
loading a complete page per request is pretty much how web pages work inherently.
you can think about frames (which execute one request per frame as well, but give you control about the reload), you can also think about caching, maybe also about some asynchronous ajax technology (buzzword alarm!).
i'm using tiles with frames, the benefit is rather low compared with a no frame solution. and frames are really old school.
i'd suggest that you dont use frames. should you run into performance problems later, you can add things like caches then.