slowlog = /var/log/php-fpm/slow.log
request_slowlog_timeout = 1s
This following two lines are added in the php config file. Once it is added and restart php-fpm, the slow.log file is created.

(request: "GET /index.php") executing too slow (1.072177 sec), logging
This error is displayed in error.log file of php-fpm. But there is no additional details.

I am not able to trace which url is causing the trouble. It is codeigniter framework. So I used the framework's benchmarking tool on all the methods and found the execution is fast 0.004 to 0.01 for multiple test

What could be the possible reason for slow.log being empty? Is there a way to get complete url in the error log for slow process?