cron jobs and speed

Evening, I hope this isn`t very stupid question (but who knows, as I have know knowledge whatsoever about the following..). I thought I`d like to know how much time each visitor spends on my site and well, I thought maybe I could get as correct answer as possible by making a cron job run in every minute, without going into uselesss details, would that slow down all the stuff a lot? If it does, how could I make it with javascript? Thanks, Siim Einfeldt