6s~10s increase in compile time

After installing `jekyll-babel`, I noticed a 6s~10s jump in compile time. I was able to isolate the performance regression to `jekyll-babel` by creating a new project and creating a single JS file, then copying it over and over.

```js
---
---
let foo = "bar";
```

Even with this simple file, there was a `+500ms` increase in compile time per JS file (4 files == `2s` increase).

Thoughts? Any way to make this faster? Thanks.

Reproduce the reported compile-time regression in a new Jekyll project with one JavaScript file, then repeat the measurement with multiple copied files. Trace the jekyll-babel conversion path to identify the per-file overhead; done means the regression is explained and a verified faster behavior or focused fix is covered by a repeatable benchmark.

Written by the indexing model from the issue text.