How to speedup bulk record deletion in postgres sql

I am having a function which delete more than 1000000 record it takes around 1 min.

CREATE OR REPLACE FUNCTION DeleteRecord (cmpId integer)
RETURNS boolean AS
$BODY$
DECLARE cmpId INTEGER DEFAULT 0;
DECLARE counter INTEGER := 1 ; 
DECLARE batch    INTEGER := 0 ; 
DECLARE batchSize    INTEGER :=50000 ; 
BEGIN

     SELECT count(*)/batchSize INTO batch FROM department WHERE cmpId= cmpId;
     WHILE counter <= batch + 1 LOOP
          DELETE FROM department WHERE deptId IN ( SELECT deptId FROM
           department WHERE cmpId = cmpId limit batchSize); 
          RAISE NOTICE 'Counter: %', counter;
          counter := counter + 1 ; 
       END LOOP ; 

     counter := 1 ;
     SELECT count(*)/batchSize INTO batch FROM employee WHERE cmpId = cmpId;
     WHILE counter <= batch + 1 LOOP
          DELETE FROM empoyee WHERE empId IN ( SELECT empId FROM
           employee  WHERE cmpId = cmpId limit batchSize);  
          RAISE NOTICE 'Counter: %', counter;
          counter := counter + 1 ; 
       END LOOP ; 

END; 
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;
ALTER FUNCTION DeleteRecord(integer)
  OWNER TO postgres;

Department table holds 500000 record for cmpId

Employee table hold more 500000 records for cmpId

I tried to delete with simple delete also but it took more than 1 min.

I found one link How do I speed up deletes from a large database table?

which tells to do delete operation in batch.

But it does helps more time reduced to by just 2 to 3 sec.