MySql LOAD DATA with subquery performance issue

I'm migrating data into a MySql (5.7.26) database (32GB ram), running as a managed service on AWS. While importing the data, I need to map one of the columns of the CSV being imported to another value using a MEMORY table lookup; so my LOAD DATA resembles the following: LOAD DATA LOCAL INFILE 'file.csv' INTO TABLE table_1(col_1, @var1) SET col_2 = (select mapped_value from table_2 where id = @var1) table_2 is a 2-column (id, mapped_value) MEMORY table with 3.4MM rows. When I import the CSV *without* the subquery, I get several million inserts per minute. However, when I run the same import with the subquery the LOAD DATA performance degrades to near zero (~100 inserts per minute). Is this to be expected with a subquery, or is there something I'm doing wrong in the example above?