30fps is optimal.  It's the same fps rate that the human eye sees with.  Sometimes games and some other apps try to increase speed by cutting it down to 28fps.   Most people can't even tell the difference.   The problem is that a certain percentage of the population (I think it's like 10% or so) have issues when the fps rate is off like that.  This is one of the things that can cause some people to get ill by watching some video games and such.