Description
Attribute indexes can speed up joins and other attribute queries on feature classes and tables. An attribute index is an alternate path used by ArcGIS to retrieve a record from a table. For most types of attribute queries, it is faster to look up a record with an index than to start at the first record and search through the entire table.
Solution
Prior to adding the join, add an index on the join field in the join table using the Add Attribute Index tool.