Unlike a normal Label, a RichTextLabel allows for the use of so-called BBCodes. Click here for a tutorial. These codes will not be visible to the user, but instead will change the appearance of the text, e.g. underlining it. In order for that to work, the text has to be "parsed" first: the computer runs over the string, looks for such codes, removes them and renders the remaining text accordingly. While doing all of this again and again whenever you're appending something to the text is possible, it's not advised.
However, given that you haven't heard of BBCode yet, this likely doesn't affect you at all. If you're using the
text-property (like it did in my example) all of this does not apply. Should you find yourself in need of BBCodes, you will need
bbcode_text instead and then - indeed - you would be better off using
I'll edit my answer to reflect this.