In the wordpress admin page edit area, it should tell you how many revisions the page has. There are 1000s of plugins to clean up revisions. WordPress by default does not set any limit on the amount of revisions.
You need to delete the post meta too, but anyway too late now.
As for the memory how much does your server have? How many sites are on the server? How much ram does the database have assigned? How many other sites share the same database? Who is the host? Is it a VPS or is it a dedicated server? Or is it shared hosting?
That might be the case. If you have not setup mysql properly it will run away with available memory. I used mysqltuner on my personal VPS which is also on digital ocean and only has 1G of ram, works perfectly fine with zero issues.
mysqltuner will look at your server and give you advice on what to change in the mysql config files based on your server size/memory. I do not know of any other tool. As the server root user it is up to you to set it up properly. A WP plugin has no control to do this for you and we are not server experts either.