Godot has a "game speed modifier." I used it with a slider in my game just because of this problem. You could use it only in dialogue.
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Ooh, yes I saw that in your game just like a few minutes ago :) I assume you mean Engine.timescale then.
I myself prefer not to use Engine.timescale, because it affects everything, and it gets a bit messy (unless you want it to affect the whole game).
I actually have a signal 'textdialogue' (which connects to another script that controls the dialogue and stuff) in an autoload with a few arguments, including the interval between the letters being added to the text. I don't really know how I never noticed the dialogues being too slow, I was thinking about being able to skip cutscenes, but I never really got the time for that.
I never really got people other than myself to playtest the game, I really should do that more often.
A slider (or just anything that allows the player to control the dialogue speed) is a great idea! If I ever continue working on the game, I'll try to add it in!
Thanks for the tip :D