extremely SLOW posts (30s)

Since yesterday, after editing 2 of my posts, they became extremely slow in rendering.
Spotted it to this plugin. Using latest versions of the plugin and Neve theme.
With all other plugins disabled, it looks like my browser gets almost whole page from the server, then waits for 25s, and then gets the footer part. Checked with curl downloading, the same.

If I enable my usual plugins, it becomes even worse, whole page doesn’t appear before 25-30 seconds. Sometimes resulting in a timeout even. I think, some plugins trying to wait for the whole DOM to be rendered.
But anyway, even just this one plugin – and pages are unusable. Other pages at the site are ok. If I try to edit and save them, they become “broken” as well.
If I “cut” parts of the page, making it smaller – it just becomes gradually faster, but it doesn’t seem like there’s a specific part of the page that introduces the slow-down, so it’s just – the bigger the page, the slower the load.

Any advice?