Have a look here
And in the comments:
If you compare a single preceding Load with a single resident Load where the two Loads have identical content, then I would say that the preceding Load always is faster. In the preceding Load, the records are evaluated "on the fly" and this makes it fast.
I made a small test where I had a Load with a few string operations. When I put them in a preceding Load, the script execution time increased by 15-20%. When I put them in a resident Load, script execution time increased by 90-100%. So, I would say: If you can put your calculation in a preceding Load, you should. But sometimes you have to use a resident Load, e.g. after a Crosstable or when you want to make some transformations after a Join.
However, a preceding Load has a small overhead, so if you have multiple preceding Loads, it will be slower. But if a preceding Load helps the manageability and clarity of the script, then I am usually willing to take a small penalty.
However i've note better performance with resident load when aggregation or lookups are needed.