### Godot version

4.0.3 stable

### System information

Windows 10, i5 8600, RTX3060, 64 GB ram

### Issue description

In Godot 4, labels with lots of text cause scenes to load extremely slowly, both in the editor and in-game.

I measured a few game starts with different amounts of text in a label:

Text length | Game start time
-- | --
11323 | 2.33
22693 | 7.93
34064 | 19.69
45431 | 40.59

In Godot 3, the startup times for all test lengths was almost instantaneous.

The text I used was multiple copies of the license agreement for Google's [Roboto](https://fonts.google.com/specimen/Roboto) font. A "credits" page on a game could easily get well above this length.
Looking at a scatter plot with a best fit, it's pretty clearly increasing at around n^2:

![image](https://github.com/godotengine/godot/assets/8082633/6e2a9c41-f3fb-429d-acd3-288c24c6b744)

Results are the same whether I'm using the built-in font or a custom font, and results are similar across all renderers. Settings like text wrapping don't seem to affect it either.

### Steps to reproduce

Create a scene, add a label to that scene, and fill the label with a large amount of text. Try to launch the game and load that scene.

### Minimal reproduction project

[Godot3LargeLabel.zip](https://github.com/godotengine/godot/files/11521494/Godot3LargeLabel.zip)
[Godot4LargeLabel.zip](https://github.com/godotengine/godot/files/11521495/Godot4LargeLabel.zip)

return jst_flags;
}
void Label::set_uppercase(bool p_uppercase) {
	if (uppercase == p_uppercase) {
		return;
	}
	uppercase = p_uppercase;
	dirty = true;
	queue_redraw();
}
bool Label::is_uppercase() const {
	return uppercase;
}
int Label::get_line_height(int p_line) const {
	Ref<Font> font = (settings.is_valid() && settings->get_font().is_valid()) ? settings->get_font() : theme_cache.font;
	if (p_line >= 0 && p_line < lines_rid.size()) {
		return TS->shaped_text_get_size(lines_rid[p_line]).y;
	} else if (lines_rid.size() > 0) {
		int h = 0;
		for (int i = 0; i < lines_rid.size(); i++) {
			h = MAX(h, TS->shaped_text_get_size(lines_rid[i]).y);
		}
		return h;
	} else {
		int font_size = settings.is_valid() ? settings->get_font_size() : theme_cache.font_size;
		return font->get_height(font_size);
	}
}
void Label::_shape() {