postgreSQL query via JDBC in different OS taking different running time?

Hi,
I want to know the reason behind the case:
My query processes from JDBC (Java Program) to PostgreSQL. I use system time by invoking java function, I collect one time unit before the query statement perform and second after the execution of query statement.
I found 85 ms time unit in DOS (win7) (laptop 4cores). both Java and PostgreSQL installed and invoked on the same machine, respectively.
On the other hand, I use same process (separate installation) on linux on 8 cores physical machine with 2times greater then laptop.
I found 150 ms. (which is a question for me because the time in Linux environment should give me half of the time taking on laptop)
I also make same setting of postgresql.conf in the linux setup, which is available same in the win7 setup, because win7 setup gives better performance of the query.
What do u suggest me, where I need to make performance tuning? which configuration setting must need to modify in the linux?
* laptop RAM 4 GB and Linux machine 32 GB
looking positive response.

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Aftab A. Chandio
PhD Scholar(Research Center for Cloud Computing)
Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences
7th Floor, Shenzhen Cloud Computing Center at National Supercomputing Center in Shenzhen (NSCS)
Xueyuan B.1068, University Town, Xili, Shenzhen, China.
+86 13244762252

Lecturer
Institutes of Mathematics & Computer Science
University of Sindh, Jamshoro, Pakistan.
+92 3003038843