to me the only why where you can earn bitcoin is gambling. to me gambling is the only way through which you can earn bitcoin in a very very fast mood.
Yes , you can earn fast on gambling but the real happening is you can also lose your money x2-x3 faster and the truth there is it is as plenty of losses than winning thats why many gamblers are broke at the end . So would you prefer to earn bitcoins very fast on gambling? If it works on you and you earn on it very fast then let me know 
It may work for you sometimes but you cannot expect that gambling will allow you to grow your coins fast every now and then in fact in long run the percentage of loosing would be higher then winning.