Magento 2: Is there a Profiler?

To trigger built-in Magento2 profiler, just add a SetEnv MAGE_PROFILER "html" to your .htaccess. You can also use "csvfile" or "firebug". In case of CSV, you'll find it in your var/log.

From v2.2.4 and above

From version 2.2.4 you can now enable/disable Profiler from the CLI :

# Enable the profiler.
bin/magento dev:profiler:enable
# Disable the profiler.
bin/magento dev:profiler:disable

Source : Magento Open Source 2.2.4 Release Notes and Magento Commerce 2.2.4 Release Notes

For older versions

If you are running on a server with nginx (with fastcgi) :

Put this code on the PHP entry point

fastcgi_param  MAGE_PROFILER  html;

Using Magento 2 nginx.conf.sample file config, you would have a node like :

# PHP entry point for main application
location ~ (index|get|static|report|404|503|health_check)\.php$ {
    try_files $uri =404;
    fastcgi_pass   fastcgi_backend;
    fastcgi_buffers 1024 4k;

    # Profiler
    fastcgi_param  MAGE_PROFILER  html;

    fastcgi_param  PHP_FLAG  "session.auto_start=off \n suhosin.session.cryptua=off";
    fastcgi_param  PHP_VALUE "memory_limit=756M \n max_execution_time=18000";
    fastcgi_read_timeout 600s;
    fastcgi_connect_timeout 600s;

    fastcgi_index  index.php;
    fastcgi_param  SCRIPT_FILENAME  $document_root$fastcgi_script_name;
    include        fastcgi_params;
}

By adding the below code to the root folder index.php works for me

   $_SERVER['MAGE_PROFILER']=1;