Informatica: connected and unconnected

There should be no performance difference between the two. However with an unconnected lookup you can use logic to determine on a row by row basis whether or not to call it. A connected lookup will be called on each row on that mapping stream. So if you don't need to do the lookup on every row AND the your session bottleneck is in the dtm you may see a performance improvement by using an unconnected lookup. Differences Between Connected and Unconnected Lookups

Connected Lookup	Unconnected Lookup
Receives input values directly from the pipeline.	Receives input values from the result of a :LKP expression in another transformation.
Use a dynamic or static cache.	Use a static cache.
Cache includes all lookup columns used in the mapping (that is, lookup source columns included in the lookup condition and lookup source columns linked as output ports to other transformations).	Cache includes all lookup/output ports in the lookup condition and the lookup/return port.
Can return multiple columns from the same row or insert into the dynamic lookup cache.	Designate one return port (R). Returns one column from each row.
If there is no match for the lookup condition, the Integration Service returns the default value for all output ports. If you configure dynamic caching, the Integration Service inserts rows into the cache or leaves it unchanged.	If there is no match for the lookup condition, the Integration Service returns NULL.
If there is a match for the lookup condition, the Integration Service returns the result of the lookup condition for all lookup/output ports. If you configure dynamic caching, the Integration Service either updates the row the in the cache or leaves the row unchanged.	If there is a match for the lookup condition, the Integration Service returns the result of the lookup condition into the return port.
Pass multiple output values to another transformation. Link lookup/output ports to another transformation.	Pass one output value to another transformation. The lookup/output/return port passes the value to the transformation calling :LKP expression.
Supports user-defined default values.	Does not support user-defined default values.