batch programming, while PBS -l nodes =2 takes long time

can any one tell me why it takes 3 mins to run the mpihello.pbs while using

#PBS -l nodes=2

cd …
mpirun -np 4 mpihello >& mpihello.log

I also notice that the output in the log file is within 2secs but to exit from the program it takes 3 mins. So can any body help me what is my problem and post a solution for me.