Very Slow Running JOIN Node

I have a table with 1.4M rows.  Three columns in the table are "code values" that must be translated into a description by joining to their corresponding ookup table. The first two columns are customer ID strings that are generally between 10-15 characters long made up entirely up numers despite the fact their data type is string.  Both of these customer lookups join to a customer table with 31,000 rows in it.  In bother cases the join node is very fast running in less than a minute. 

The third lookup node is my issue.  It mastches a customer group column in the source table that is generally between 15-20 characters of mix case alpha-numeric values to a lookup table that contains only 22 rows.  I would have expecetd this one to significantly outperform the fist two nodes but its the exact oposite.  This join takes 3-4 hours to complete.

I have done everything I can to ensure the data types are correct like stripping off trailing spaces, converting both to upper case, etc.  Nothing seems to improve the performance of the join.  I tried changing memory settings to use disk, use memory and only use small tables in memory.  I get the same results.

Is there anything you could think of to tune where the smaller join would perform as well as the faster ones?

For 'joins' with small lookup tables you can/should use the cell replacer node. It's sometimes an order of magnitude faster as the lookup table is small (<1Mio) and the join can be done in one scan.