Category Page loading issue, if that page have sidebar "search/engine" tracking.
Step 1: Check Catalog Search Default Search Engine options under catalog menu and enable Mysql Engine

Step 2: Check Other Catalog Search Extensions Modules
Example: Advanced Search,Auto Complete Search Extensions
Step 3: Update the 'core_config_data' table 'catalog/search/engine' path value using below code
UPDATE core_config_data set value = 'mysql' where path='catalog/search/engine' // As default its mysql2