this is too few information. Which database is on output? How is set commit size for output.
And you can try use bulk loader step. It's available for some DBMS, ex. PostgresSQL, MySQL.
There are difference between DB join and lookup. DB join call query for every row of stream. But DB lookup can use memory cache for key fields or load whole table to memory.
And performance depends od executed lookup query of DBMS. You can create some optimisation, ex. sort data for lookup fields and use cache. Query is executed only for first row, next rows by same key fields check cached lookup result at first or execute query.