At the end of the day, the theme size determines the speed of the theme itself, but then from there you are going to add in plugins which will likely have a much larger effect than the theme itself.
Personally, because I build my pages with Elementor, I use the Hello Theme by the Elementor team because it is literally a blank theme -- it loads virtually nothing and if you don't use a builder like elementor with it, it looks awful lol. However as long as you use a builder that allows you to generate your own header and what not, it's about as lightweight as you can possibly get.
Seriously though, if you're exclusively focusing on the themes rather than the plugins, you're probably putting the cart before the horse. Plugin usage will slow things down far more than your theme will; you can use a "slow" theme but very few plugins and wind up with a faster site than if you used a "fast" theme with a lot of resource-heavy plugins.