Thanks for your reply @mazedulislamkhan. Here are the answers to your questions:
Does this issue occur while using the WordPress v4.9.4 (latest)?
Yes.
Does the issue occur while all the non-Yoast plugins are disabled?
Yes.
Does the issue occur while the default 2017 theme is active?
Untested.
Is there any specific posts or pages where the issue occurs? Or does it occur on all over the site (backend, frontend)?
All over the site. Slow page generations times were shown in our cache performance log. They were typically 50-70% higher than normal.
If you create a staging site on your localhost by copying the production site, does this resolve the slowness issue?
I do not have the capabilities to do this.
It seems many other people are experiencing similar problems.
For me, downgrading to 6.3.1 stopped the issues completely.