WordPress article with 50 000 words loading slow – 100% cpu

Check the number of post revisions for that one 50K word article. While in Edit Page, go to Screen Options and be sure “Revisions” is ticked.

If you have more than one revision, WP is querying the database for that one large post and all revisions, and that will account for some – possibly all – of the CPU spike. The CPU spike is MySQL working on the query to display the article.

Delete some or all of the page revisions. See https://wordpress.org/plugins/revision-control/

And because you have a VPS, try https://github.com/major/MySQLTuner-perl to adjust RAM and cache and other parameters for MySQL. Those can make a huge difference in MySQL performance.