Choose a fast theme that’s not full of bloatware, delivering unused CSS and JavaScript to the browser for every single request.
That’s probably a theme that doesn’t come with a page builder bundled into it.
GeneratePress
For the time being this site is using GeneratePress. It seems good enough. If you don’t believe me measure the performance of this post.
Free themes
There are over 9,200 themes on wordpress.org. A very small set of them are Full Site Editing (FSE) themes. FSE themes are also known as block based themes. Each of the themes on wordpress.org has been through a theme review process. They should be technically good. Picking a pre-built theme is easy. Switching between themes didn’t used to be easy. But it’s getting easier.
Background
When I started preparing the performance measurements I initially picked the Avada theme, but gave up since I don’t have an active licence. I reverted to Twenty Twenty-Two; we were using the site for WordPress 5.9 demo.
But I wanted a theme with a small header, otherwise the mobile performance would be pretty good, since that’s all you get to see on a Moto G4 screen. So I picked GeneratePress.
Twenty Twenty-Two and FSE themes
If you wish you can preview this post with a number of other themes.