Performance related to size of tables

Hi everyone,
I found that performance get worse as the size of a given table
increases. I mean, for example Ive just run some scripts shown in

http://www.potentialtech.com/wmoran/postgresql.php

I understand that those scripts are designed to see the behavior of postgresql under different filesystems. However, since them generate
a lot of I/O activity, I think they can be used to adjust some
configuration parameters. In that way, I increased the number of tuples inserted in the initial table to 2000000 and 3000000. What
I saw is that the running time goes from 3 min., to 11 min. My question is, is it possible to use that test to tune
some parameters?, if the answer is yes, what parameters should I change to get shorter running times?

Thanks a lot

Nestor