Here several weeks I slowed to a site. It's been several days since I research the origin. It seems to me that this is the plugin "pop over".
I optimized the database, cleans.
With the P3 plugin, I verify that I increase the access of more than 2s time by activating the plugin.
Can you check what does not work ?
I have given access to the site (https://premium.wpmudev.org/forums/topic/a-i-want-a-popup-each-time-a-member-is-logged-but-for#post-634535)