Isn't it a much better idea to pick a distance comparison algorithm that's not so staggeringly slow to begin with? Why not convert (i.e. pre-compute) the lat/lons to 3-vectors (origin center of the earth) and then compare distances by pythagoras, no fuss, muss or trig and you can even skip the square root. If you need to display an actual accurate distance you can do the (significantly smaller amount of) trig to get something presentable.