The problem is that TTF fonts do not have any caching mechanism for the glyph they once rendered. Every time you request a certain string from a TTF label, this exact string will be rendered to a texture and then displayed as a sprite.
Bitmap fonts definitely solve that, they can be batch rendered. But bitmap fonts have other issues - you can’t provide a bitmap font for every language and all symbols needed in all languages - or at least it’s hard.
It’s funny that you did this up now. I’ve suggested/asked yesterday to create bitmap fonts from TTF fonts at runtime. Maybe that would also solve your issues (once we have a a solution):