From the tutorials I watched, the person made a new layout and sprite sheet for each level. If I have 15+ levels and each one has a layout and a sprite sheet, does that slow the game down?
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Images are loaded into memory per layout. In games with large levels and/or lots of artwork, video memory is often the limiting factor.
As far as rendering/processing speed, you can also separate your event sheets per layout, so you don't run through a bunch of events that don't apply to the current layout, but I believe the performance effect is negligible/irrelevant.
Edit: The biggest factor is the initial download time/size of your game - if you have tons of layouts and assets, those will all need to be downloaded when you first load your game.