what is the difference between j’ai descendu rapidement and je suis descendu rapidement

Does this link help?  Descendre can be used with avoir or être in compound tenses depending on its meaning in French (Le Passé Composé)   

 J’ai descendu is a transitive use of the verb: it will always have a direct object, so your first phrase is incomplete (nothing is being taken down). Je suis descendu is intransitive, so it can be used as a standalone phrase or be followed by a preposition. The discussion under the lesson is also worth a read.

 (rapidement is a bit of a red herring and has no bearing on whether to use avoir or être)