How to Index a JOIN result of two or more tables in order to improve the performance in SQL server?
A foreign key does not have an index by default. You have to create one.

In this case, I'd suggest either of these. It depends on relative selectivity of the 2 columns

CREATE INDEX IX_table2fk ON
              table1 (t2_col1, t1_col3)
              INCLUDE (any columns needed for the query)


OR

CREATE INDEX IX_table2fk ON
              table1 (t1_col3, t2_col1)
              INCLUDE (any columns needed for the query)


The INCLUDE will make the index covering to avoid key/bookmark lookups