Well, the main difference is that document.ready() event gets called as soon as your DOM is loaded. It does not wait for the contents to get loaded fully. For example, there are very heavy images on any web page and takes time to load. If you have used window.onload then it will wait until all your images are loaded fully, hence it slows down the execution. On the other side, document.ready() does not wait for elements to get loaded.
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