subroutine do_fast()

  implicit none

  integer :: i, x
  
  do i=1, 100000000
     x = i*x
  end do

  print *, "Done with fast."

end subroutine do_fast
  
subroutine do_slow()
  
  implicit none

  integer :: i, j, x

  do j=1, 4
     do i=1, 100000000
        x = j*i*x
     end do
  end do
  
  print *, "Done with slow."

end subroutine do_slow

program example

  implicit none

  integer :: i

  do i=1, 10
     call do_fast()
  end do

  do i=1, 5
     call do_slow()
  end do

end program example