The cube was there so that I could test object portaling so if you push the cube through one of the seamless portals it will portal seamlessly. The performance issues/bugs were a lot more prevalent in the 0.6 update before I mostly fixed the performance issues by disabling portals when not in view. But, there are still game-breaking performance issues when running the game on lower-end hardware. This would probably be mostly fixed with a resolution setting in the settings menu. The furniture needs collision I just haven't gotten to adding it for every piece of furniture yet. If you want a little 'behind the scenes' on how the game works, you can check out my devlog:
https://24k9robot.itch.io/perdition/devlog/495823/bugperformance-fixes