add_filter( 'big_image_size_threshold', function( $threshold ) {
    return 1200; // max width in pixels
}, 10, 1 );

Resize and compress the images

// Set max image width to 1200px
add_filter( 'big_image_size_threshold', function( $threshold ) {
    return 1200;
});

// Set JPEG compression quality to 80%
add_filter( 'jpeg_quality', function( $quality ) {
    return 80;
});

add_filter( 'wp_editor_set_quality', function( $quality ) {
    return 80;
});

Convert PNG → WebP (smaller size + transparency support)

add_filter( 'wp_editor_set_quality', function( $quality, $mime_type ) {
    if ( $mime_type === 'image/webp' ) {
        return 80;
    }
    return $quality;
}, 10, 2 );