hi!

it depends on your data and the type of operation u r doing.

If u need to calculate a value for all the rows or for the maximum rows coming out of the source then go for a connected lookup.

Or,if it is not so then go for unconnectd lookup.

Specially in conditional case like,

we have to get value for a field 'customer' from order tabel or from customer_data table,on the basis of following rule:

If customer_name is null then ,customer=customer_data.ustomer_Id

otherwise  

customer=order.customer_name.

so in this case we will go  for unconnected lookup

1.can u explain one critical mapping?

Ans: Reading from multiple sources and writing to multiple targets using number of TLP's and applying complicated business rules then it is a challenge to every ETL developer  to balance the complexity with not sacrificing performance issues.
2.performance issue which one is better? whether connected lookup tranformation or unconnected one?
Ans: The fundamental difference between connected and unconnected lookup is the number of return port values. if you need multiple field values from a lookup you have to use connected, and if you need only one port value from the lookup then you can use unconnected, and also unconnected can by used any other transformation by calling it.

For Example:

You need salary of an employee then you can use unconnected lookup. But if you need salary and department of the employee then you have to use only connected lookup.

And remember the more lookups and the more performance issues.