I’ve been looking more into the Godot GUI lags on tree change issue with the help of a friend in Godot 4.2.2. and we’ve managed to track down the root cause to the RichTextLabel taking a long time when it is first drawn.

RichTextLabel uses another class to draw text called TextParagraph. Consider the example below, where TextParagraph is used directly:

@tool
class_name CachedTextLabel
extends Control
 
@export_multiline var text: String:
	set(value):
		if text != value:
			text = value
 
			var font := get_theme_default_font()
			var font_size := get_theme_default_font_size()
			paragraph = TextParagraph.new()
			paragraph.add_string(text, font, font_size)
 
			queue_redraw()
 
var paragraph: TextParagraph
 
func _draw() -> void:
	var start := Time.get_ticks_msec()
	paragraph.width = size.x
	paragraph.break_flags = TextServer.BREAK_MANDATORY | TextServer.BREAK_WORD_BOUND
	paragraph.justification_flags = TextServer.JUSTIFICATION_NONE
	paragraph.alignment = HORIZONTAL_ALIGNMENT_LEFT
	paragraph.draw(get_canvas_item(), Vector2.ZERO)
	var end := Time.get_ticks_msec()
	print("_draw: ", end - start, "ms")

Normally, the draw performance is fine. However, also consider that we want to render the following text (like I want to do in no signal):

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Under these conditions, the first draw call takes about 100ms on my machine to run (and about 300ms) on my laptop. However, subsequent draw calls take only 1ms. It seems like TextParagraph is lazily loading in some data when the first draw call happens.

We can proactively trigger whatever it is that TextParagraph would otherwise do before the first draw call…

func _ready() -> void:
	paragraph.get_size()

…and this fixes the lag that happens on the first draw call.

So, it looks like the performance problem isn’t an issue with tree changes, but rather something that the RichTextLabel does when rendered for the first time.