As Len mentioned, placement of the speed nocks is critical. Essentially there is a sweet spot you have to find where you will get the maximum speed gain. There are also places you can find on a bow where they will actually slow it down (as you found out).

The only way to get them in the correct spot is lots of time shooting through the chrono.....lots of times.

Start with a baseline speed of your rig.
Install the speed nocks a little lower than you would expect to have them and shoot a couple of arrows.
Move one of them up toward the cam until you see a peak in the speed followed by a decrease for further movement.
Move the nock back to the position for the highest speed attained and repeat with the one on the other end.

All in all you can gain the speed they say....in most cases, however it takes lots of time and patience to find your individual bow's sweet spot.