@mohdaljaraba
The easiest way to find out whats causing the issue is to profile one request using xdebug and analyze that, but its going to take a lot of setup first.
In the meantime, you can simply install plugin named Query Monitor
It will show you which PHP files are consuming time on your system the most, just go through its screenshots, and FAQs section and learn to use that plugin.
Don’t forget to deactivate that plugin when done, because, it consumes CPU cycles and RAM as well.