OK, I also tried this on Windows 10, the same hardware (dual boot). The game runs better on Windows, but still it's demanding for such a simple game. The controls are really too unresponsive, I think. And I can't get past stage two, it's too tight and just barely touching anything makes the rocked disappear and makes us start all over again - so scraping the walls is inevitable here and should be part of the mechanics, with some kind of deformation and destruction design to add realism and a challenge. As on Kubuntu, there is no way to open any kind of menu during gameplay, even to quit, but I did manage to kill the app with ALT+F4, which on Kubuntu doesn't work, as I said before.
Hello Andrew, first of all, thanks for playing my game. I'm really grateful that someone played my game. OK, so as you mentioned, controls are unresponsive. I made the controls in this way so that the player doesn't have the time to react/think while thrusting in the air, so that the game doesn't feel very easy or unchallenging. I want players to grind a little to get used to the controls. As you said, you couldn't pass level two, but I want to mention that I was able to complete each and every level during the playtest, so I don't know what's happening with you( I'm not being rude here). I'm going to upload a video soon where I'll show each and every level from start to finish, so it might help you a bit. Also, if I face any difficulty or find a specific part making the level harder to complete then I'll change it asap. At last, I want to thank you again for taking the time to comment, and I'll add the pause screen and other features in the future when I get the time, as I'm really new to game development, so these things might take some time to figure out.