If you want to improve your GTmetix score Defer Parsing of JavaScript, you can simply copy the following codes and add it to your functions.php file. You would not need any plugins to do so.

Here are other tutorials to improve your GTmetrix Score:

• To improve Add Expire Headers & Leverage browser caching: Click here
• Cache configuration as recommended by GTmetrix: Click here
• To serve Scaled Images & Optimize Images: Click here