I saw Altrive's video on this game on YT (congrats on the publicity btw!) and some things in this game didn't look right so I went ahead and played it for myself.
There are 2 glaring issues with this fangame, and 2 QoL features that would make it better:
1. As some comments pointed out, the natural sun production speed does NOT speed up when you speed up the game at the top right. The tutorial level is literally impossible to beat if you press 2x speed, making it an unneccessary handicap rather than a QoL feature.
2. The bloons damage sound is not capped, so if there are too many bloons on screen (lets say, 100 bloons) that means that ALL those bloon sounds are going to stack and make extremely loud sounds. This would be a problem because on later levels the volume would start to hurt your ears if you wear headphones. You need to make some sort of max volume setting.
3. Speaking of settings, adding a volume slider in the settings would be nice. (This one is less of a problem and more of a missing QoL feature)
4. The bloon movement speed feels a little too fast, even on normal speed mode. This makes it so that if you make one tiny mistake you can be severely punished and immediately lose a lane along with every plant in that lane. You can see instances of this in AltrivePlus's video at 22:53 where he loses an entire lane from ONE SINGLE pink bloon at top lane. I think you should either increase the sun production speed so that the plant side can keep up with the bloons, or you should slow the entire bloon populations' movement speed by 20% so that a bloon rush consisting of 2 rainbow bloons is more manageable.
I'm not really a game designer (I mostly study programming shittily), so I can't really comment on the overall design direction of your fangame, so I think you just need to make the game more forgiving and easier to beat overall so that the players can get to the cool parts of your game easier and not have to sit through 2 hours trying to figure out how to beat night 3.