I don't use React-based static site generators, but I'd imagine if they become slow it's because it's generating a large code-base, but unless you provide some benchmarks showing compilation of React vs some other templating system I can't really take your argument against it for this use case.
The developer experience is better. I personally like type safe templates, and just the JSX bits of React are one of the most mature type safe templating frameworks with very good editor support.
most template engines have partials and some have macros (Jinja2)
React-based static site generators are so slow that they’re unusable for any website that has more than a couple pages