Monstra CMS - Fast and small flat-file content management system written in PHP!
Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
Fansoro - Fansoro is a Modern Open Source Flat-File Content Management System.
TYPO3 - TYPO3.com - Infos, SLAs, Extended Support Versions and more
Grav - The modern open source flat-file CMS
ClassicPress - The WordPress fork. No Gutenberg. Great future!