I built small hot lava planet (radius 400) close to sun but not too close, with 3 hot lava moons (radii between 100 and 150) orbiting it on close orbits (but not colliding with a planet or each other nor the sun). When you preview the third one (regardless of the order of previewing) or when you build this system, you get incredible simulation slow down (10 fold or greater), in at least 90% of cases. Rarely previewing all the planets with moon or building the system doesn't generate any slowdown. This means my hardware configuration can handle the editor well but there is some bug that causes slowdown in most cases. (you have to preview or build the system 4 planets or planet with 3 moons) for the slow down to occur in most cases.