Use this plugin

https://wordpress.org/plugins/code-snippets/

to add this small code:

add_filter('wpforo_content_after', function( $content ){
    return preg_replace_callback(
'#<a*\sclass=(?:*\s)?wpforo-default-attachment(?:\s*)?*\shref=(+)*>.*?</a>#isu',
        function( $match ){
                $html     = $match;
            $file     = $match;
            $pathinfo = pathinfo( $file );
            if( wpforo_is_image($pathinfo) ) {
                $html = sprintf(
                    '<a href="%1$s" target="_blank"><img class="wpforo-default-image-attachment" src="%1$s" alt="%2$s" title="%2$s"></a>',
                    esc_url($file),
                    esc_attr($pathinfo)
                );
            }
            return $html;
        },
        $content
    );
}, 11);

I would recommend Fastest Cache or Automaticc's WP Super Cache. Whatever works for you.

And still, exclude wpForo from them.

No way. I would recommend to use SEO Framework instead, that works with wpForo.

Or if you do not like Seo Framework, use YOAST.