Loop, query, and PHP time treatment

Hello again

My theme is close to finition.
But I do have some worries.

Instead of one main loop, I needed two separate colums and thus used “query” for both of my columns :

<?php query_posts('cat=-6,-25&paged='.$paged); ?>
<?php if (have_posts()) : ?>
etc

and

<?php query_posts('posts_per_page=18&cat=6,-25&paged='.$paged); ?>
<?php if (have_posts()) : ?>
etc

When my blog has hundreds or thousands of posts, will these queries make a huge difference in terms of ‘time treatment’ ?

Will I be facing a slower and slower treatment because of the number of posts queried ?

Or are there “safeguard parameters” such as the number of posts per page (main settings) which will ensure there is no drift ?

Thanks