Set JPEG Compression Quality

Changes the quality used when WordPress generates resized JPEG images. Lower values give smaller files; the default is 82.

Code

add_filter( 'wp_editor_set_quality', function ( $quality, $mime_type ) {
    return 'image/jpeg' === $mime_type ? 75 : $quality;
}, 10, 2 );

Applies to images uploaded after activation. Use a thumbnail regeneration tool for existing images.

Install WPIDE on your WordPress site (it is free) and open Code Snippets.
Use snippet above adds it from the library in one click. Or copy the code into a new snippet with the same code type.
It is added inactive: review the code and settings, then switch it on.