Performance tuning in sql storeproc

Question

I am dealing with Huge data almost 5 million data in a table.and the table having 7 columns composite primery key because itese columns should not have duplicate value.using the procedure if the record is exit then am updaing the data else inserting the data in table.this procedure is working fine but performace is very slow as query covrage is on multiple column index. Kindly help me is there any othet way to improve performnce.