CSS menus based on lists formatted as table cells are pretty fast. However, whenever there is a button image download, it takes more time.
BV menus are a bit slower, because each button has it's own pair of images. A 10 button menu has 20 images, so even if they are small, it is always some bytes to download. With css instead, you only have two (or three or four, depending on your menu) images, that are the same for all buttons. HAve a look at the menu
here which is css.