Similar problem.
The Plesk admin page is very slowly too. But sometimes the speed is tolerable.
14 low traffic, simple wordpress website.
Often only the first (website) load is slow. (admin page always slow)
CPU: Intel(R) Xeon(R) CPU E5-2690 v4 @ 2.60GHz (8 core(s))
Plesk: Plesk Obsidian...