"Is there anyway to know the exact reason for Mage delay? do I have other test i can use to identify the cause of delay?" Ultimately, yes. Easily, probably not.
First of all, see if you can replicate this issue in a staging or development environment. This will allow you to experiment without risking a negative impact to the production site.
It sounds like you are on the right track. You need to slowly eliminate possible sources of error, for example disabling extensions as you did. Other things I would try:
ensure codebase is at same version as production
ensure database is the same, as closely as possible: same stores, same products and categories, same system configuration
just disable your caches so you are not tricking yourself. Later, after you have resolved the issue, you can enable the caches in order to improve performance.
clear magento cache:
rm -rf var/cache var/full_page_cache
reindex from the command line:
php shell/indexer.php reindexall
After these basic cleaning steps, go through the workflow during which you experience slowness. If you still do not experience the slowness, then you can be reasonably confident that the issue is related to the server configuration: mysql, apache, DNS, redis, CDN, anything external to the Magento application. If you do experience the slowness, then that is a sign that something is amiss with your instance of the Magento application: a code issue, and extension, incorrect admin configuration, something of the nature.
I hope that helps. Unfortunately, with such a general error, I can't provide much more assistance than these basic troubleshooting steps to try and determine the source: Magento itself? or an environment issue?