```python
from fastapi import FastAPI, Query
import asyncio
import helpers  # noqa: F401 (the provided API)

app = FastAPI()


@app.get("/portfolio")
async def get_portfolio(symbols: str = Query(...)):
    # Split comma-separated symbols and strip any surrounding whitespace
    symbol_list = [s.strip() for s in symbols.split(",")]

    # Launch all price fetches concurrently to avoid blocking the event loop
    tasks = [helpers.afetch_price(sym) for sym in symbol_list]
    prices = await asyncio.gather(*tasks)

    # Map each symbol to its fetched price
    price_map = {sym: price for sym, price in zip(symbol_list, prices)}
    return {"prices": price_map}
```