It would be useful to specify in the documentation that, when submitting tasks, arguments are passed by value by default (see std::bind()), which can significantly slow down the pool.

And that one should use std::ref() to pass arguments by reference, such as:
auto my_future = pool.submit(task, arg1, std::ref(arg2));

It would be convenient if there was a way for arguments to be passed by reference automatically when the function definition specifies it, but not sure how to do it.