I agree with Roger, and learned this advice directly from Tom House. Preset some pronation of your changeup pitch or else it just comes out like a mediocre fastball, which does you no good whatsoever.
Your fastball is fast because you release it palm-forward, toward the target, and your index and middle fingers are directly behind the ball, efficiently providing force to it. If you release a palm ball in exactly the same palm-forward way as your FB, you will be putting three fingers directly behind the ball. Why does anyone think this would slow the ball down? Lots of people will tell you "bury the change-up deep, that will slow it down"--but, guess what? Doing that doesn't slow the ball down enough to make a good changeup....
If you cannot seem to master the art of releasing your change-up with preset pronation, then try using the split-finger fastball as your "change-up".
You do not need to take special pains to pronate a splitter--it is thrown palm-forward just like any other fastball. Because the index and middle fingers are "split" to the sides of the ball, they are not in a position to efficiently provide force to the ball at release. So the "splitter" grip automatically takes off speed even when it's delivered with normal FB mechanics. Splitters also spin more slowly than typical FBs so there is less spin-induced lift on these pitches--so, they sink more for two reasons: Because they travel slower to home plate than FBs, and also because they have slower backspin than typical FBs.