I'm having a bizarre issue with my site that started happening about a week ago.
It's a multisite and the main site of the installation takes about 10/15 seconds to respond for logged in users. Afterwards it loads fine. If you log out then everything loads normally.
The sub sites all load normally either way.
The subsites run the same plugins as the main site. A couple of them also run the same theme and they work fine too.
Also this problem just started happening without any changes or updates having been made.