I've been looking for how to do them in several pages, but in all the ones I've found I see that they use a specific time in which the pre-loader will appear, and my question is if the function of the preloader is to appear for a specific time or appear in a varied time to the internet connection of the user waiting for the last image to finish loading ? since I see that mega.nz uses a preloader before loading the file. And adding another question is how effective are the preloader?