It could be many things. Your preview browser might not be able to handle the level size, there may be an object that is constantly being created without being destroyed(an overload of object spawn), it might be that your game is constantly saving(saving game progress) without having the objects, that are not needed to be saved, have the "No Save" behavior(because there are so many objects in the game, or instances of an object), it could be that your level is just too big, or maybe your computer is just too slow. It shouldn't really be that your computer is too slow, unless it is kinda old, or just very low-end, because usually even low-end devices can run the projects very well. It could also be that your sprites have too large of a resolution.
Hope that helps! Let me know how it goes, and if you still can't figure it out, maybe screenshot the level so I might be able to help a bit more.