It basically comes down to computer specs and how heavy your 3D scene is. All that geometry and shaders and stuff on screen takes up memory and resources. Specially if you have too much geometry to the point that is wasteful. We start with simple scenes when we are learning Blender and we don’t pay attention to computer resources, because all we do is simple stuff and we slowly build up in complexity, and it is then when we hit a wall, we max out our computer resources and then we have to start thinking how to work around limitations. So congrats, you’re leveling up, you’re doing more complex things now.