It doesn't matter how much time the Green Lantern themselves experience. However, the nature of crisis is that if you don't get there fast enough from the objective time of whoever is experiencing the crisis, it's not going to help. If some evil invader is about to destroy all the major cities of another planet ID4 style, then if the GL can't get there in good time from the frame of reference of the planet in distress, they're not going to do any good as a space cop corps. The GL Corps needs to be able to cover sector wide distances quickly, or at least with very little delay in objective, such that if there is any time dilation, it should be minimal at those distances.