This link explains the following about the pyqtSlot decorator:
Although PyQt4 allows any Python callable to be used as a slot when
connecting signals, it is sometimes necessary to explicitly mark a
Python method as being a Qt slot and to provide a C++ signature for
it. PyQt4 provides the pyqtSlot() function decorator to do this.
and
Connecting a signal to a decorated Python method also has the
advantage of reducing the amount of memory used and is slightly
faster.
Since the pyqtSlot decorator can take additional argument such as name, it allows different Python methods to handle the different signatures of a signal.
If you don’t use the slot decorator, the signal connection mechanism has to manually work out all the type conversions to map from the underlying C++ function signatures to the Python functions. When the slot decorators are used, the type mapping can be explicit.