If you have enabled
Batch Processing on any Blocks in your Map, best practice is not using a Lookup Block.
Lookup Block and
Lookup Functions do not support Batch Processing. Lookups significantly reduce the performance of Batch Processing, because they must retrieve information one record at a time from the selected entity. The diagram below outlines the progress of a Map that contains both a Lookup Block and an Update Block with Batch Processing enabled. Steps 2 through 5 are performed one record at a time, which slows down the process of integrating data into the target.