I have created a new category on my store after creating this website taking 15-20 seconds to load, but before that it's taking only 5-6 seconds
Make sure reindex is not invalid and check enabled or not.
If the help you then mark as solution.
You could enable the Magento's profiler (this guide will help you https://docs.nexcess.net/article/how-to-enable-the-magento-profiler.html )
Or you can use this one https://github.com/AOEpeople/Aoe_Profiler too. This is a great tool to find bottlenecks.
I installed AOE profiler and get a query which causing slow website
SELECT `main_table`.`entity_id`, main_table.`name`, main_table.`path`, `main_table`.`is_active`, `main_table`.`is_anchor`, `main_table`.`url_path` AS `request_path`, `url_rewrite`.`request_path` FROM `catalog_category_flat_store_1` AS `main_table` LEFT JOIN `core_url_rewrite` AS `url_rewrite` ON url_rewrite.category_id=main_table.entity_id AND url_rewrite.is_system=1 AND url_rewrite.store_id = 1 AND url_rewrite.id_path LIKE 'category/%' WHERE (main_table.is_active = '1') AND (main_table.include_in_menu = '1') AND (main_table.is_active = '1') AND (main_table.path like '1/2/%') ORDER BY `main_table`.`position` ASC
but when i remove newly created category it's goes back to normal as previous state like loading in 5-6 sec.
That time is not right.
What if you execute that query into the mysql server just to see if the bottleneck is the database or the code?