Good and slim web CMS software (à la Wordpress)?

I set up a WP website some time ago for a relative. It is basically just a bunch of static pages, the only functionality WP adds is editing, which is essential.

It has served its purpose well, but said relative is unable to self-manage the site without bloating it or slowing it down to a crawl. I have no intention of managing their website, so I'm interested in a platform (free preferred, paid is also fine) that would allow them to modify content on their site / rearrange pages in menus etc. without giving them the possibility of messing up too much.

Thanks for reading.