Ah. You do not have actually go from high performance to high quality. High performance is a preset for setting everything to low, while high quality is a preset to set everything to high. I feel this has something to do with antistrophic filtering. Nifflas, you might be able to tell us what kind of rendering techniques are used for rendering text?
He's actually referring to a different setting than what you're thinking of, although that may affect this setting as well. You seem to be thinking of the general performance slider that effects a number of different settings at once, like AA, AF, etc. There's a separate setting for texture quality as well though. I use ATI Tray Tools, where it's called 'Texture Preference'. From what I understand, this setting has practically no performance impact on modern graphics hardware, so it's best to leave it set to high quality anyway.
I've heard of the setting causing blurry text on some hardware in other games as well, including some commercial titles, though in that case the graphics drivers may get updated to fix that. People have said elsewhere that setting this 'Texture Preference' setting, along with 'MipMap Detail Level' to high quality fixes the problem, though it's possible that only one of them causes it.