Against any given pitcher, the timing of the stride has to be same every pitch so that you can be on time with the fastball. If you are striding to hit the fastball, then how do you adjust to an off-speed pitch?
The ability to adjust to changes in speed is determined by the position achieved when the stride foot lands. Adjusting is only possible with a strong base and pressure between the knees. The clips below show the difference in Barry Bonds' swing hitting a fastball and hitting an off-speed pitch. Notice how in both swings he controls his move forward by pulling his hands rearward with his back as he strides.
Fastball
Off-Speed Pitch
For the fastball, he attacks the ball immediately after the stride foot lands. For the off-speed pitch, he delays the hands, but continues forward and sinks into his bent front leg. Because of the resistance provided by keeping his knee inside his front foot and the hold of the hands with his back, he is able to keep his head and upper body back and stable. Even though some weight has already shifted to the front leg, the shift is controlled and he remains balanced over his rear leg. This gives him a strong platform to drive the baseball from when it finally reaches the hitting zone.
To handle multiple speeds, you must "control your weight" in order to "control your wait". You control your weight by being in a strong and stacked position when the stride foot lands with your hands back, your rear hip coiled, your head between your feet, and pressure between your knees. Keep that coil and the pressure between your knees on your way to the ball, then attack the backside of the baseball.