SQL & PL/SQL

I have an insert...select query where the select is a join between a table with one billion rows (although there is a where clause on an indexed column that restricts it to "only" 300 million), a table with 30 million rows, and a table with about 100,000 rows, where the result is about 20 rows. When I first ran it, it took about 2 hours. I added a Parallel hint, and explain plan showed that it was being used (and v$session showed that I had about 30 additional connections while it ran). but not it takes four hours.

Is there a reason parallel processing would cause a query to run slower?

insert /*+ append */ into employees_by_age_group
(
  pay_plan
, age_range
, pay_level
, fy
, employee_count
)
select /*+ parallel */
       emp.pay_plan
     , to_char(d.min_age) || '-' || to_char(d.max_age) as age_range
     , emp.pay_level
     , pay.fy
     , count(pay.employee_id) as employee_count
from 
(
  select /*+ index(pay_info pay_info_index_on_site) */
         employee_id
       , extract(year from (dte_ppe_end + 92)) as fy
       , count(employee_id) as num_recs
  from pay_info
  where extract(month from dte_ppe_end) = 10
  and   extract(day from dte_ppe_end) between 14 and 27
  and   substr(pay_type, 1, 1) IN ('A', 'B', 'C')
  and   site like 'Z%'
  group by employee_id, extract(year from (dte_ppe_end + 92))
) pay
join
(
  select employee_id
       , pay_plan
       , pay_grade
       , pay_step
       , file_date
  from 
  (
    select /*+ index(employee_info employee_info_index_on_site) */
           employee_id
         , pay_level
         , file_date
         , max(file_date)
           over (partition by extract(year from (file_date + 61)))
           as last_file_date
    from employee_info
    where site like 'Z%'
  )
  where file_date = last_file_date
) emp
on (
     emp.employee_id = pay.employee_id
     and extract(year from emp.file_date) = pay.fy - 1
   )
join (
       select employee_id
            , dob
       from (
              select employee_id
                   , date_birth
                   , row_number() over (partition by employee_id order by date_file desc) as r
              from employee_birthdates
              where site like 'Z%'
            )
       where r = 1
     ) dob
on dob.employee_id = pay.employee_id
join
(
            select 20 as min_age, 24 as max_age from dual
  union all select 25 as min_age, 29 as max_age from dual
  union all select 30 as min_age, 34 as max_age from dual
  union all select 35 as min_age, 39 as max_age from dual
  union all select 40 as min_age, 44 as max_age from dual
  union all select 45 as min_age, 49 as max_age from dual
  union all select 50 as min_age, 54 as max_age from dual
  union all select 55 as min_age, 59 as max_age from dual
  union all select 60 as min_age, 64 as max_age from dual
  union all select 65 as min_age, 69 as max_age from dual
  union all select 70 as min_age, 74 as max_age from dual
  union all select 75 as min_age, 79 as max_age from dual
  union all select 80 as min_age, 84 as max_age from dual
  union all select 85 as min_age, 89 as max_age from dual
  union all select 90 as min_age, 94 as max_age from dual
  union all select 95 as min_age, 99 as max_age from dual
) d
group by emp.pay_plan, d.min_age, d.max_age, emp.pay_level, pay.fy;