Given a correct function that converts a JSON-representable value to embed-safe ...

Given a correct function that converts a JSON-representable value to embed-safe JSON, you can use it on the JSON to get your JSON.parse performance:

  const inlineJSON = data =>
    JSON.stringify(data)
      .replace(/\u2028/g, '\\u2028')
      .replace(/\u2029/g, '\\u2029')
      .replace(/</g, '\\x3c');

with:

  const escapedReduxStateJsonString = inlineJSON(JSON.stringify(data));

But yeah, the isolated <script> thing is usually even better (more compact in addition to the security benefit).