The processing clip algorithm has been optimised for common use scenarios, resulting in dramatic speedups for clipping operations. For example:
Clipping a roads layer with 1 million lines against 2 polygons
Earlier versions: 5 mins 30 seconds QGIS 2.18: 10 seconds
Clipping a 5 million point address layer against 2 polygons
Earlier versions: 50 minutes QGIS 2.18: 30 seconds