Does that slow down the game? Adding a sprite per letter seems like a real resource hog. Also, why is this preferable to simply typing text in paint and then using that picture as a sprite? Then all the text would only be one sprite.
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SpriteFont use only one texture (a letter sheet) and it's all encompassed in one object. There's no real overhead I think. Under the hood It just draws some rectangle picking the corresponding texture area for each letter.