Question about WebSocketConnectionPtr::send

Hi,
I'm wondering if `WebSocketConnectionPtr::send` is a blocking method?
This is of an interest since I'm interested in understanding how would a websocket server implemented with Drogon cope with sending large amount of data to multiple connections, some are slow and some are fast?
If the `send` method does not block and can be queued on any i/o thread in the event loop, is there a way to tell when it completed?
Thank you in advance.