Optimize this Php call?

I got a little code that calls all the posts in the category of the currently viewing post

it is as followes

<ul>
<?php
foreach( ( get_the_category() ) as $category ) {
$the_query = new WP_Query('category_name=' . $category->category_nicename . '&showposts=-1');
while ($the_query->have_posts()) : $the_query->the_post();
?>
			<li>
				<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>" rel="bookmark"><?php the_title(); ?></a>
			</li>
<?php endwhile; ?>
<?php
}
?>
</ul>

some categories have 500+ posts, and this is a very slow query

anyway to make it more efficient? or at least make it load last or separately so it does not affect the overall loading time of a page?