[mhagemeister]

The performance of type inference matters for runtimes which work with TypeScript files directly, rather then using the transpiled .js + .d.ts files like Deno. This has several benefits in that we can jump to the source on "go to definition" rather than some random .d.ts file among other things. Deno uses the original TS files for type checking as well and that's why inference also affects type checking performance in some runtimes.

By ensuring explicit return types in the public API, the generation of .d.ts files is turned into a mere syntax transform, rather than requiring the tsc compiler. This is something TypeScript will ship with in the next release itself. They're adding a new `isolatedDeclarations` option which is the same thing.