Here 不久 and 很快, their meanings are roughly the same. Both of them mean "after a short time". The reason why you feel confused is because the word "快" has several meanings in Chinese. The general 3 meanings of "快" 1. Fast, quick/quickly (in terms of the speed of movement) e.g. 马跑得很快。 The horse can run very fast. 2. After a short time. In the near future. e.g. 晚饭很快就好了。 The dinner will be ready soon. 3.Happy,delighted. e.g. 我很快乐。 I am happy. Therefore, "很快"can mean both very fast and very soon.