When should I use どんどん and when should I use 速く?


Both are adverbs meaning "quickly", but what is the difference of use between
them ?




どんどん implies that you progress step by step.  
速く does not include this nuance.

From
<https://www.google.com/amp/s/www.weblio.jp/content/amp/%25E3%2581%25A9%25E3%2582%2593%25E3%2581%25A9%25E3%2582%2593>

物事や動きの切れ目がなく、次から次と続くさま。




どんどん implies that you progress step by step.  
速く does not include this nuance.

From
<https://www.google.com/amp/s/www.weblio.jp/content/amp/%25E3%2581%25A9%25E3%2582%2593%25E3%2581%25A9%25E3%2582%2593>

物事や動きの切れ目がなく、次から次と続くさま。

