Firstly, the old codebase is TS namespaces, which compile down to IIFEs that push properties onto objects. Each file that declares that namespace is its own IIFE, and so every access to other files incurs the overhead of a property access.

With modules, tooling like esbuild, rollup, can now actually see those dependencies (now they are standard ES module imports) and optimize access to them. In this PR's case, the main boost comes from scope hoisting.

For example, in one file, we may declare the helper `isIdentifier`. In namespaces, we would write `isIdentifier` in another file, but this would at emit time turn into `ts.isIdentifier`, which is slower. Now, we import that helper, and then esbuild (or rollup) can see that exact symbol. All of the helpers get pulled to the top of the output bundle, and calls to those helpers are direct.

That's why modules gives us a boost. There's also more (modules means we can use tooling to tree shake the output, and smaller bundles are faster to load), but the hoisting is the big thing.