To quote the Tai Chi Classics: "If the opponent's movement is quick, then quickly respond; if his movement is slow, then follow slowly."
Some of the training methods (i.e. form) are slow; this is to develop strength, accuracy, balance and control.
New skills are often slowed down for safety until the student gains familiarity.
As the student becomes adept, the kung fu speeds up considerably.
Combat is usually fast.