jsonb value retrieval performance

Hello:

Does JSONB storage has some internal indexing(like hasmap)
to fast look up a value given a key?
I have a jsonb doc with two level keys
(parentKey: {childKey:value}}
there are maybe 2000 parent keys per doc and 100 child keys per parent key

and I am trying to get value via jsonb->parentKey->childKey
it seems it is very slow.
Would it be actually faster to use top level key only and parse it at
client side?

Thanks