Please look at the theme sources. If there is used WP_Query - then you should pay high attention to its params. By default wordpress adds to the query a string to return the count of all rows if only few are returned (for paging, etc).
In the places where you do not need this please add this line to the params list:

'no_found_rows' => true,

so your arguments will look like:
$args = array(
'posts_per_page' => get_option('pst_on1st_page'),

'no_found_rows' => true,

'cat' => get_option('cat_to_show')
);

that line of code will extremely increase database performance.