For me, Se often comes fastest, at least when I am paying attention to my environment, or something in the external world I want to observe. Te then follows fairly quickly, even if only to render the judgment, "you need to gather more data". Determining the speed of my dominant function, Ni, is very difficult. Ni perceptions can appear as "flashes of insight", which sounds like something fast, but they are often the product of hours, days, even weeks of subconscious rumination on an "everything else" that I will not attempt to define. On the other hand, sometimes they really are fast, prompted directly by something that has just happened. I suspect that feelings (the experience of emotion, not the Fe/Fi functions) come faster than anything, but they usually sneak under the radar, and don't come to my attention unless they are unusually strong, or I make a deliberate effort to examine them.