If the Magento 1 backend is extremely slow, even when logging in or saving products, there are a few tricks that can help. Two key problem areas are the /var/cache directory and the core_url_rewrite table.
If the backend is already extremely slow when logging in, it can help to manually delete the contents of the var/cache/ directory.
If editing or saving products is very slow, this may be caused by slow database queries. These can be identified, for example, using a slow query log. In our case, the culprit was the extremely large core_url_rewrite table, where queries sometimes took more than 30 seconds to complete.
The solution was to back up the manual rewrites and select them, for example, using a REGEXP:
SELECT * FROM `core_url_rewrite`SELECT * FROM `core_url_rewrite`WHERE (NOT (`id_path` REGEXP '^[0-9]') AND `is_system` = 0) OR (`is_system` = 1 AND `options` = 'RP')ORDER BY `core_url_rewrite`.`is_system` ASC
Then export the results, empty the table, import the manual rewrites, reindex (e.g. via the CLI using php shell/indexer.php --reindex catalog_url), and clear all caches.
And just like that, everything is fast again! Encountered at our web agency in a Magento online store running CE 1.9.
