Cache the output of a plugin

Hi WPMU Dev's.

On my homepage on I have the latest post displayed from our Pro Sites members, but the plugin pulling the posts is really slow, so I need some way of caching it or optimising it.
The developers of the plugin says it is slow because of the sites large database (about 5GB), but I still don't think the homepage should take 40 seconds to load - please note that this is only happening when logged in, as WP Rocket cache is caching when users are logged out.

I think there is 2 ways to make it load faster.. Optimise the plugin or cache the plugin on the page, and make it preload every 30 minutes.
I really need some help here..