MPI_Recv takes a long and strange time to return - openmpi

Question

I am trying to compare the execution time of two programs:
-the first one uses non blocking functions MPI_Ssend and MPI_Irecv, which allows to do some calculations "while messages are being sent and received".
-the other one uses blocking OpenMPI functions.
I have no problem to assess the performances of the first program and they look "good".
My problem is that the second program, that uses MPI_Recv, often takes a very long and kind of "strange" time to finish : always a little bit more than 1 second : (for example 1.001033 seconds). As if the process had to do something that takes exactly 1 second.
I modified my initial code to show you an equivalent one.
#include "mpi.h"
#include <stdlib.h>
#include <stdio.h>
#define SIZE 5000
int main(int argc, char * argv[])
{
MPI_Request trash;
int rank;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
int myVector[SIZE];
for(int i = 0 ; i < SIZE ; i++)
myVector[i] = 100;
int buff[SIZE];
double startTime = MPI_Wtime();
if(rank == 0)
MPI_Issend(myVector, SIZE, MPI_INT, 1, 1, MPI_COMM_WORLD, &trash);
if(rank == 1) {
MPI_Recv(buff, SIZE, MPI_INT, 0, 1, MPI_COMM_WORLD, NULL);
printf("Processus 1 finished in %lf seconds\n", MPI_Wtime()-startTime);
}
MPI_Finalize();
return 0;
}
Thank you.