signal/slot在底层会使用三种方式传递消息。参见QObject::connect()方法：
bool QObject::connect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method, Qt::ConnectionType type = Qt::AutoCompatConnection )
最后一个参数是就是传递消息的方式了，有四个取值：

Qt::DirectConnection
When emitted, the signal is immediately delivered to the slot.
假设当前有4个slot连接到QPushButton::clicked(bool)，当按钮被按下时，QT就把这4个slot按连接的时间顺序调用一遍。显然这种方式不能跨线程（传递消息）。

Qt::QueuedConnection
When emitted, the signal is queued until the event loop is able to deliver it to the slot.
假设当前有4个slot连接到QPushButton::clicked(bool)，当按钮被按下时，QT就把这个signal包装成一个 QEvent，放到消息队列里。QApplication::exec()或者线程的QThread::exec()会从消息队列里取消息，然后调用 signal关联的几个slot。这种方式既可以在线程内传递消息，也可以跨线程传递消息。

Qt::BlockingQueuedConnection
Same as QueuedConnection, except that the current thread blocks until the slot has been delivered. This connection type should only be used for receivers in a different thread. Note that misuse of this type can lead to dead locks in your application.
与Qt::QueuedConnection类似，但是会阻塞等到关联的slot都被执行。这里出现了阻塞这个词，说明它是专门用来多线程间传递消息的。

Qt::AutoConnection
If the signal is emitted from the thread in which the receiving object lives, the slot is invoked directly, as with Qt::DirectConnection; otherwise the signal is queued, as with Qt::QueuedConnection.
这种连接类型根据signal和slot是否在同一个线程里自动选择Qt::DirectConnection或Qt::QueuedConnection

这样看来，第一种类型的效率肯定比第二种高，毕竟第二种方式需要将消息存储到队列，而且可能会涉及到大对象的复制（考虑sig_produced(BigObject bo)，bo需要复制到队列里）。