One of my stored procedure has long execution time (average around 4 to 7 minutes).

Now I trying to tweak it an make it run faster.

I am looking at execution plan and two things I see that using most of percentage.
First is 68% of "Clustered Index Scan" of one main tables for reporting, This table has primary key of two columns and 2000000 records inside.

Second most demanding task is 26% of "Merge join" (left outer join), this is connection between already  mentioned table and other with foreign key to one of two columns of primary key in first table.

I checked Index physical statistic and index for these tables are in good conditions.

What can I do to increase performance of this stored procedure.

Do I need to make new index on this tables.

Does `set transaction isolation level read uncommitted` statement help to improve performance ?