For me (firefox 3.6.15 on XP), the live preview of my answer was painfully slow, to the point where it stopped working altogether. I went to faketestsite and the preview worked perfectly, and also looked very different (I could take screenshots if I knew how to place them here).

I do recall that at some point in time a while back, the look of the texed part of this site changed, and simultaneously the load time increased. This may be relevant.