When you open the main page with type "category blog", there are query without limits, which runs through all the records in table of com_content (_content), which creates high load on the database.

If set type of main page menu item as "Blog of category" it generate SQL querry without limits.

File: \components\com_content\models\articles.php

Function: function getListQuery()

Example of querry:
Query_time: 60.808180
Lock_time: 0.000460
Rows_sent: 46159
Rows_examined: 1245897

SET timestamp=1376672974;
SELECT a.id, a.title, a.alias, a.title_alias, a.introtext, a.checked_out, a.checked_out_time, a.catid, a.created, a.created_by, a.created_by_alias, CASE WHEN a.modified = 0 THEN a.created ELSE a.modified END as modified, a.modified_by, uam.name as modified_by_name,CASE WHEN a.publish_up = 0 THEN a.created ELSE a.publish_up END as publish_up,a.publish_down, a.images, a.urls, a.attribs, a.metadata, a.metakey, a.metadesc, a.access, a.hits, a.xreference, a.featured, LENGTH(a.fulltext) AS readmore,CASE WHEN badcats.id is not null THEN 0 ELSE a.state END AS state,c.title AS category_title, c.path AS category_route, c.access AS category_access, c.alias AS category_alias,CASE WHEN a.created_by_alias > ' ' THEN a.created_by_alias ELSE ua.name END AS author,ua.email AS author_email,(
SELECT MAX(contact.id) AS id
FROM qitw0_contact_details AS contact
WHERE contact.published = 1 AND contact.user_id = a.created_by) as contactid,parent.title as parent_title, parent.id as parent_id, parent.path as parent_route, parent.alias as parent_alias,ROUND(v.rating_sum / v.rating_count, 0) AS rating, v.rating_count as rating_count,c.published, CASE WHEN badcats.id is null THEN c.published ELSE 0 END AS parents_published
FROM qitw0_content AS a
LEFT JOIN qitw0_content_frontpage AS fp ON fp.content_id = a.id
LEFT JOIN qitw0_categories AS c ON c.id = a.catid
LEFT JOIN qitw0_users AS ua ON ua.id = a.created_by
LEFT JOIN qitw0_users AS uam ON uam.id = a.modified_by
LEFT JOIN qitw0_categories as parent ON parent.id = c.parent_id
LEFT JOIN qitw0_content_rating AS v ON a.id = v.content_id
LEFT OUTER JOIN (SELECT cat.id as id FROM qitw0_categories AS cat JOIN qitw0_categories AS parent ON cat.lft BETWEEN parent.lft AND parent.rgt WHERE parent.extension = 'com_content' AND parent.published != 1 GROUP BY cat.id ) AS badcats ON badcats.id = c.id
WHERE a.access IN (1,1) AND c.access IN (1,1) AND CASE WHEN badcats.id is null THEN a.state ELSE 0 END = 1 AND (a.catid = 18 OR a.catid IN (
SELECT sub.id
FROM qitw0_categories as sub
INNER JOIN qitw0_categories as this ON sub.lft > this.lft AND sub.rgt < this.rgt
WHERE this.id = 18 AND sub.level <= this.level + 1)) AND (a.publish_up = '0000-00-00 00:00:00' OR a.publish_up <= '2013-08-16 17:07:51') AND (a.publish_down = '0000-00-00 00:00:00' OR a.publish_down >= '2013-08-16 17:07:51')
ORDER BY CASE WHEN a.publish_up = 0 THEN a.created ELSE a.publish_up END DESC , a.created;

Type: "category blog"
In Category options: Subcategory Levels - 1

In Blog Layout Options:
Leading Articles - 0
Intro Articles - 16
Columns - 2
Links 5
Include Subcategories - 1

I have database with ~46 000 of Articles after export from old CMS (there are news portal), 95% of articles in 1 category and 7 sub categories.

If item type not main page there are correct query with limits.