WordPress functions.php tricks: Disable image compression
WordPress automatically compresses JPEG images at 90% quality. In this article, I will show you how to increase or decrease WordPress JPEG image compression.
All you need to do is paste the following code in your theme’s functions.php file or your site-specific plugin.
add_filter('jpeg_quality', function($arg) { return 100; });
There are definitely performance benefits to leaving the compression quality as is. If you want, you can change the number from 100 to 80 or something lower to squeeze a few more kBs.