I really think what happens especially with taking supplements, is that you end up supplying your hair follicles with the nutrients it was lacking and therefore retarding your normal growth rate and keeping your hair from being as healthy as it can be. So you simply just bring your growth rate to the optimum (read: normal) rate. I try to eat well and have taken a potent multi since the beginning of the nineties plus my mom was anal about ensuring we had wholesome balanced meals growing up. So when I joined the forum and went on all the supplements that folks claim speed up growth, they made no difference to me. They were simply a waste of money since for instance my diet was already replete w/ biotin. I returned to my normal way of eating without all those and my hair growth rate did not slow down.
As for topical products, I also think they make a difference if they treat a skin condition you have. I am yet to see evidence of sped up growth that defies normal hair growth because of them.
But as someone said, do what feels true to you. Sometimes the proof is in the pudding.