```python
from fastapi import FastAPI
import helpers
import asyncio

app = FastAPI()


@app.post("/reports")
async def create_report(payload: dict):
    rows = payload.get("rows", [])
    # Offload the synchronous, blocking render_report to a thread pool
    rendered = await asyncio.to_thread(helpers.render_report, rows)
    return {"length": len(rendered)}
```