Revisit React SSR performance optimizations

[onlywei]

Background

There was an experimental branch (react-performance) with changes claiming ~20% performance improvement for @fastify/react SSR. The branch was incomplete and stale, but the ideas are worth revisiting.

Optimization Ideas

1. Sparse serialization in toJSON()

Only include properties that have values, reducing payload size:

// Instead of always including all properties:
toJSON() {
  return {
    actionData: this.actionData,
    state: this.state,
    data: this.data,
    // ...
  }
}

// Only include properties with values:
toJSON() {
  const json = {}
  if (this.actionData) json.actionData = this.actionData
  if (this.state) json.state = this.state
  if (this.data) json.data = this.data
  // ...
  return json
}

2. Hook consolidation

Move preHandler logic into onRequest to reduce hook overhead - fewer async boundaries.

3. Use renderToString for non-streaming SSR

When streaming isn't needed, renderToString may be simpler and faster than renderToPipeableStream.

4. Use native PassThrough over Minipass

Remove the minipass dependency in favor of Node's native PassThrough stream.

Next Steps

• Benchmark current SSR performance as baseline
• Test each optimization individually to measure impact
• Implement optimizations that show measurable improvement