For the last few weeks, I've been wondering why when I try to load many pages in the admin, the browser will say "Waiting for subsite1.mymultisite.com..." then it will go through nearly every sub-site on my multisite network. This often takes as long as 30+ seconds, as there's quite a few sites on my Multisite.
I've been dreading having to go through and deactivate all the network activated plugins to find out which one was causing this. I realized Acunetix WP Security was the most recent plugin I had added and network activated. So I network deactivated, and suddenly my sites are all a whole LOT faster! No more running through some kind of loading from each sub-site to load the network admin, or even individual site's admins.
It was such a pain: I'd go to edit or update a post and it would just sit... 10, 20, or 30 seconds running through all the other sub-sites before it could complete loading the post editor page. Now with Acunetix WP Security disabled, it all works great. NOTE: I had the live traffic feature turned off, so it wasn't that. Or at least not any part that's disabled when it's turned off.
I really like this plugin, and would love to continue using it. But you'll have to further optimize it for Multisite, as it's current state slows big sites down. Please prevent it from needing to pull/load whatever it is currently trying to do, from each individual sub-site. There's no need and it makes using a large Multisite unbearable for all users.