If the rate is one turn in 10 inches then that is what it is spinning as it leaves the barrel. If you speed it up or slow it down it still is spinning at that rate when it leaves the barrel within a given distance. If you increase or decrease the velocity then the RPM or RPS will increase but so will the distance traveled in a given time frame. Two different animals however, velocity in a given twist with regards to bullet length does affect stability.

Here is a tool from Berger that will help with your understanding if you take time to play with it.
http://www.bergerbullets.com/twist-rate-calculator/