I'm finding that it's easier to write static analysis tools for language X in language X. It's less to think about.
I've been writing an analysis for JS. I really like working with typescript but it's been easier to prototype if I dogfood.
I'm rewriting a Rust function transform function from `&mut Foo` to `Foo -> Foo`. It might be a little slower, but ownership makes it harder to miss parts of the Foo type when I work with values.
I've seen 'modularity shaming' for JS libraries that are big and make sites slow to load. I'm surprised to see Wikipedia start reporting on app size though!