Magento uses a lot of resources to re-index data from the Admin Panel. A good way to re-index data without using the admin panel is to use command shell “ssh”. This also would work if you happen to be locked out of the admin panel.
Here is instruction on how to use default Magento indexer via SSH:

1 – Go to ‘shell’ folder in your Magento installation:

cd shell

2 – Execute reindex using this command:

php -f indexer.php reindexall

It is possible to get full list of Magento indexer commands using this command:

php -f indexer.php help

To get a list of available indexes, execute this command:

php -f indexer.php info

By default there are 8 indexes in Magento:

catalog_product_attribute     Product Attributes
catalog_product_price         Product Prices
catalog_url                   Catalog Url Rewrites
catalog_product_flat          Product Flat Data
catalog_category_flat         Category Flat Data
catalog_category_product      Category Products
catalogsearch_fulltext        Catalog Search Index
cataloginventory_stock        Stock status

Individual commands for re-indexing:

php indexer.php --reindex catalog_product_attribute
php indexer.php --reindex catalog_product_price
php indexer.php --reindex catalog_url
php indexer.php --reindex catalog_product_flat
php indexer.php --reindex catalog_category_flat
php indexer.php --reindex catalog_category_product
php indexer.php --reindex catalogsearch_fulltext
php indexer.php --reindex cataloginventory_stock
php indexer.php --reindex tag_summary