If you use a font rendering engine like FreeType, then the most common mistake is to re-render strings every frame. This is pointless, because when you render a string, it will usually be on the screen unchanged for at least a few seconds.

When you want to display a text, render that text to a texture once and then draw that texture instead of re-rendering the text.

When you do this, the cost of text-rendering will only apply the first time you need to render a given string with given settings. Afterwards the cost is the same as for any other sprite.