Hi Mewity and Vortex!
Stages load the most amount of content and may stress some lower-end laptops or PCS out if they don't have the RAM. Try picking the debug stage which has less to render and see if it works.
We are planning to optimize further by unloading everything that is not in the players view to simplify stages. Currently, enemies do dissapear when off-screen but we want to extend this to stage chunks and props too.