Also, wow. I just noticed that this was a beginner submission, very well done.
My guess is that if the movement is done via code, it might not be limited by the frame rate. If you've written the code in the _process() method, try switching it to _physics_process(). I don't use Godot as my main engine, so I'm not completely sure... If you have already done that or if the bubble movement is an animation, then the issue might be somewhere else. It is also possible that I'm experiencing this because of my weird setup. I have a decent 144Hz laptop which is connected to a 60Hz monitor, so that might affect the frame rate and the game's performance on my end. I hope you find the issue! I'll finish the game when it's fixed!