Riding nationals has always been a timed event. If some one acts in a hurry he is probably running behind for whatever reason. The slower rider had better yield or the other rider will more than likely pass without regard to the slower riders misfortune. You normally have a 30 minutes grace which you are given extra points for each minute. Also many a tie breaker can come down to the fastest time. I beleive Lampkin won the Scottish last year based on the faster time due to a tie of points.
I was initiated to this at my first Ute Cup. Being a lowly `newbie` expert rider I was passed on a log and had my bars knocked out of my hands. Luckily a photographer came along and removed my bike off me. I rarely get passed ever since. Also my first national was so ill timed most riders timed out the first loop, something like 70% timed out that day.
I usually make sure the slower rider is aware I am there. After that it is his problem!