Interesting game! Creative gameplay. There are lots of options for completing levels.
I can see how to get to the other 10 levels, but it wasn't immediately obvious. I only found this out after completing the first level, wondering if there was more than one level and then pressing tab when it restarted. And then it's only possible to switch level when you've restarted the one you're in.
Thanks for the warning about lag spikes. Unfortunately this constituted about 30-40 seconds of unresponsiveness the first time, then 5-10 seconds on a subsequent load of the page. It also caused the entire browser to become unresponsive for a few seconds at one point during that subsequent load which was a little worrying.
I had a similar problem in my game where I wanted the level to slowly fade from blue sky/green grass to yellow/red, and I was regularly updating the world lighting every 5 seconds or so. Unfortunately this caused momentary lag spikes every few seconds, and I thought that for all I know, it might be even worse on other computers, so I completely removed the feature, taking away a lot of the atmosphere from the game.
If you make a similar game in future (or update this one) I suggest a similar approach where you disable shaders, at least by default. As the issue only occurs when first moving the ball, it seems that it's just the shaders for the sparks and trails, which add visual flair but are not 100% required to get the point across. Then if you want, you can provide the option to re-enable them in the menu. If the issue doesn't exist in the Windows build then that could have them on by default.
I'm curious about how you did the ball rolling sound - is it just controlling the volume on a simple sound effect or is it more dynamic? And shaders - I haven't looked into Godot shaders but was there any special framework you used / assets / libraries you downloaded for the effects?
Sorry for the wall of text.