What karlis said is that the system doesn't annule your record when your fps go below 30 one time. It annuls your record when you are below 30 fps for X amount of time.
And I don't believe a warning should be necessary, if you have fps spikes, there's nothing you can do about it during the race, and if there is (for example closing a demanding program), you lose time doing it so the rec won't be good anyway.