the_excerpt() in WordPress is too slow

I have many posts in my WordPress blog.

These posts are very long and have not <!--more-->.

So I have to use the_excerpt() function to create excerpt of post automatically.

But the time cost is too long.

I render top ten posts using about 8 seconds.

I try to print the the_excerpt() execution time like below.

<div class="description">
    <?php timer_start(); ?>
        <?php the_excerpt(); ?>
    <?php echo timer_stop(); ?>
</div>

The time is between 0.8s~2s.

It is so strange…

So how can I improve the speed of page rendering?

Thank you all!