On the WP Forums (at https://wordpress.org/support/topic/multisite-with-one-media-library) you posted this comment:
Ipstenu (Mika Epstein) Half-Elf Support Rogue, Volunteer Forum Mod & Plugin Referee Posted 1 year ago # http://wordpress.org/plugins/network-shared-media/ Keep in mind, this is generally something that kills your site’s performance.
Unfortunately, the thread is closed so I can’t ask you to elaborate through the forum. So can you elaborate via email reply on exactly where or how the “performance is killed” as a result of installing & activating the `Network Shared Media` plug-in? Thank you, -Doug
The problem is that WordPress Multisite was built to be multiple separate sites. You used one install of WordPress, one install of all your themes and plugins, and one ‘user base,’ but outside of that, the sites are all separate. Which plugin is active is different, which theme is used is different, and most importantly your data is segregated.
Your data includes your media.
The concept of ‘sharing media’ is a great one. If you have a network, you would want to have a way to share all common media in a way that every site can call it and insert it. But the way most plugins (including the one mentioned above) go about it is by doing sql queries:
$blogs = $wpdb->get_results( $wpdb->prepare("SELECT blog_id, domain, path FROM $wpdb->blogs WHERE site_id = %d AND public = '1' AND archived = '0' AND spam = '0' AND deleted = '0' ORDER BY registered DESC", $wpdb->siteid), ARRAY_A );
The code, in and of itself, isn’t bad, but that’s just to get a list of sites on the network. Then you have to check if each user has access to upload files to the site. Then you have to use
switch_to_blog to go to each site and load the images.
What happens if you have a lot of sites and a lot of images?
Your site is slow.
We’ve made a lot of improvements on that end, but extensive DB queries when you have to cross paths like that can be ‘expensive’ on a computational level. Not to mention those large DB queries can cause headaches if you use database caching. All that data has to be generated and cached on every load. That too will slow your server down.
Now this won’t always make your site slow, but if you use it and you end up with a slow site, I’m not surprised.
The next logical question, of course, is do I have suggestions or alternatives? I don’t. I actually do suggest people use that plugin if they need shared media. Or I suggest they don’t use Multisite.