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

Question

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

Answer 1

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。