This is a comment made by u/aninteger on reddit

Since you are using asio I think you really need to be writing async handlers. You can't just set multi-threaded mode on and write handlers that do blocking calls because Crow doesn't appear to be a http server library setup with a large thread pool.

And he/she is correct. Crow does not have non blocking handlers. It would also be a nice feature to have, I'll have to look into it however since I don't even know where to begin working on this.