Admin Portal Very Slow when Editing / Saving Posts

Hi

I have a new issue with WP whereby if I edit a post and try to save it, my resource usage goes through the roof (from about 10% to 100% immediately, especially for CPU) crashing my admin and site for people online.

How can I get to the bottom of understanding what is causing that spike? I have tried to use query monitor etc but that does not show data when the crash happens on editing a post.

Basically what happens is I press ‘the update post’ button and then the ‘Saving CSS’ messages appears, says ‘CSS Saved’ but then it hangs before the ‘Post Updated’ message and then (normally crashes my site).

Thank you.