DB access[Read/Write] slow

Our Mongo DB version : 2.4.3
Mongo Java driver : 2.11.3


process1:
slaveok=false

we are reading and writing to db1, the load is approx 100 hits/ seconds.

process2:

slaveok=false
we are reading and writing to db2, the load is approx 10 hits/ seconds.

But whenever the process1 is running process2 also taking more time to respond.

Note : All collections in db1 and db2 are not sharded.Each db has 40 collections and maximum document for a collection is 400,000.

My question is that DB level lock may slow down the collection owned DB [db1], but why the db2 based process is getting slow?

And also Please help me to take the longterm solution for this problem?

Shall I shard all collection in db1 and db2?
Shall I make the slaveok=true? [but it'll give stale data]?
Does Mongo db upgrade give any solution for this?