You don't need to think that it is "either" punch fast "or" punch hard. If you practice the 'center-line" punching technique you'll be punching with your feet legs and hips and moving your hands/arms very little, and a guy maybe 3x faster than I am but if he moves his hands 4x more than me...Try hiting the double end bag with real punches- extend your jab arm 100%, so on-- but only as it comes to you and don't "throw" punches at it- drop your right and barely raise your elbow to hook. Done right before long you'll be hitting a peanut bag with a jab left upper left hook combo, all good blows.