MPI Isend/Irecv Bottleneck

Dear all,

I have some questions related to MPI Isend/Recv bottleneck.
Below is my subroutine named "Broadcast_boundary":

      DO NP=1,NPROCS-1
        IF(MYRANK==NP-1)THEN
         CALL MPI_ISEND( ARRAY_1D(L/2-NUMBER),NUMBER,MPI_REAL,NP  ,101,MPI_COMM_WORLD,IREQ1,IERR)
         CALL MPI_IRECV( ARRAY_1D(L/2)       ,NUMBER,MPI_REAL,NP  ,102,MPI_COMM_WORLD,IREQ2,IERR)
         CALL MPI_WAIT(IREQ1,STATUS1,IERR)
         CALL MPI_WAIT(IREQ2,STATUS2,IERR)
        ELSEIF(MYRANK==NP)THEN
         CALL MPI_ISEND( ARRAY_1D(L/2)       ,NUMBER,MPI_REAL,NP-1,102,MPI_COMM_WORLD,IREQ1,IERR)
         CALL MPI_IRECV( ARRAY_1D(L/2-NUMBER),NUMBER,MPI_REAL,NP-1,101,MPI_COMM_WORLD,IREQ2,IERR)
         CALL MPI_WAIT(IREQ1,STATUS1,IERR)
         CALL MPI_WAIT(IREQ2,STATUS2,IERR)
        ENDIF
      ENDDO

The code is designed to communicate the boundary data between np-1 and np from 0 to nprocs.
And here is my sample program:

      L=20000; NUM=500
      ALLOCATE(A(L),B(L))
      CALL RANDOM(A)
      CALL RANDOM(B)

      CALL MPI_BARRIER(MPI_COMM_WORLD,IERR)
      TIC=MPI_WTIME() 
      CALL BROADCAST_BOUNDARY(A,NUM)
      TOC=MPI_WTIME()
      MPI_WTIMES(1)=TOC-TIC
   
      CALL MPI_BARRIER(MPI_COMM_WORLD,IERR)
      TIC=MPI_WTIME() 
      CALL BROADCAST_BOUNDARY(B,NUM)
      TOC=MPI_WTIME()
      MPI_WTIMES(2)=TOC-TIC

As far as I am concerned, the mpi_wtimes(1) (the elapsed time to communicate an array A) and mpi_wtimes(2) (array B) will be nearly same because the size of A and B are equal.
But after several experiments, I found that mpi_wtimes(1) took about eight times more time than mpi_wtimes(2).

Please let me know if there's an initial process in MPI communication, or if there's something on the first communication that accelerated the next performance, or if my code needs to be improved.
I'll attach the entire code I tested. If you can test it and let me know the results, I think many questions will be answered.