Grav - The modern open source flat-file CMS
Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
WonderCMS - WonderCMS is the smallest flat file CMS. Completely free, open source, privacy oriented for over 12 years.
Anchor CMS - Free and lightweight blogging system
HTMLy - HTMLy is an open source databaseless blogging platform. A Flat-File Blog and Flat-File CMS written in PHP prioritizes simplicity and speed.
Textpattern - Textpattern is an elegant content management system that is free, open source software.