What does all this mean?
Why use MPI_ISEND on short messages? MPI_Ixxxx primitives must allocate a request handle for you, which is not free.
If you can guarantee the receive is posted, use MPI_[I]RSEND. This bypasses the handshake.
Most MPI's are not threaded internally so MPI_ISEND just defers the transfer to MPI_WAIT