For playing fast every approach that focuses on "finger strength" "heavy pressure" "very hard", squeezing stress balls etc is just plainly wrong and counterproductive, as it suggests that you need to make efforts, more efforts, and again more efforts, train your muscles until they are hard, torture your fingers until they are strong, very strong and independent.Forget it. Think of your health.Speed comes from release and let go. You have to learn to fly with your fingers. You have to learn the appropriate movements. These will result from a very subtle and playful cooperation of your musical imagination with your fingers, arm, wrist and everything else that belongs to your playing apparatus.Yes you should practice slowly! But not with an emphasis on strength but on precision and lightness.