The Preload ad option will instruct the player to preload the VAST/VPAID XML template and thus prepare your ad content to be served faster. As these XML files are usually small in size, the impact of this option will most likely be small but when it comes to speed and ad display, every millisecond counts!
NOTE: For ad preloading to work correctly on mobile, the player will preload parts of your video beforehand as this is the only way for this option to work on mobile devices. This is a limitation of displaying video ads on mobile devices as they require a video to be loaded into a video slot before an advertisement can start playback.
The amount of video preloaded is decided by the browser itself.