Tai Qi and Kung Fu are the Yin and Yang of Chinese martial arts. They are both physically challenging disciplines and the fundamentals that they require, develop and refine are the same - balance, coordination, suppleness and strength. They differ only in the particular approach each employs. Tai Qi focuses on slow, steady movements and a soft or relaxed outer posture as the primary teaching tool to develop inner strength (bones, muscles, tendons) while Kung Fu employs fast, dynamic, energetic movements as the means to develop inner softness and suppleness.
As a practical matter, the pace of all of your forms (Tai Qi or Kung Fu) will initially be the same (or very close to it) - relatively slow for Kung Fu and relatively fast for Tai Qi. As you practice, the pace of your Kung Fu will increase as your inner core relaxes and the pace of your Tai Qi will decrease as your inner core is strengthened.
Give some thought to practicing both of these styles as a single element - practicing both of them at fast and slow pace in order to gain a full appreciation of your body and its movements.