I dont have to write data at all .
I know other disadvantages of nested data in Firebase.
Just wants to know, will it affect my performance even if I access data directly by their key values, if yes then how?
Depth is not a factor that affects speed on a technical level. Firebase can as quickly look up a node at level 30 as it can at level 1. The by fast major contributing factors to performance are the size of the data your app reads/writes and the available bandwidth.
But years of experience with Firebase has shown that deeply nested data often goes hand in hand with performance problems. It's not a causal relation, but often developers pick nested structures that are simply the wrong data structure for their apps use-cases.
This really depends on the data structure and the way you access it. Which makes this question way too broad in it's current form. For good introductions to the topic, I recommend reading NoSQL data modeling and watching Firebase for SQL developers.