Using an innocent way like this:

@app.route('/my/url/big.data')
async def my_big_data(content_type='application/octet-stream', **server):
    buffer_size = server['context'].options['buffer_size']

    with open('/my/folder/big.data', 'rb') as f:
        chunk = True

        while chunk:
            chunk = f.read(buffer_size)

            yield chunk

normally will not block the event loop. As long as the blocking io call open() and f.read() is quick (ie. You have a fast drive).

The subsequent calls to it (inside while chunk/True) is really safe. Thanks to the download speed limiter.

Internally, Tremolo will suspend each chunk / yield at some amount of time depending on the given speed setting. The lower speed setting given, the more chance for other coroutines to run.

Using a library like awaiter should gives better performance.