Impact on script execution time

[eluser]Aniket[/eluser]
what is the impact of having multiple

Code:

$db1=$this->load->db('xyzdb',true);

my requirement is m connecting to same database and executing at least 20 "select" queries with different "where" parameters using active record class.
will it have impact on execution time of the script ?
I have set my scripts execution time to

Code:

set_time_limit(0);

I profiled the script on localhost and it takes around 4 seconds...but what if i execute on a live website ?
Actually i am writing a newsletter application which contains many different topics and is sends to more than 8000 users ?

Thanks in advance