Hello
i have a similar problem. No 404 error though, no slow backend.
But all images, CSS and JS files result in error 500.
Which obviously makes Wordfence unusable on a multisite installation.
However, this seems to be due only to the way the .htaccess in the plugin folder is written.
Still trying to find out how to correct the order directive which seems to be causing the problem on my GNU/Linux Apache webserver.