You may try the below approach.
--> Use DRS to replicate SFDC object into a FF Target
--> You can use the above FF as a lookup object instead of SFDC object in the mapping.
--> Since your source is as well Flat File and all the files would be in Secure Agent location, this would run faster as well.
You can also use the look up SQL override for relational databases, Hence as Nithy has suggested you can either use the FF approach or extract the salesforce details to a relational tables using replication task and then using the combined sql query you can use the SQL override to look up the data.
You can use pushdown optimization in this case to improve the performance.
Please refer below: