The speed at which you should make a turn when driving depends on the turn you're making.
If you're gonna be turning at an intersection or other street in a residential area then you should come to a complete stop before turning.
Then slowly when safely able to make the turn at around 10 to 15 MPH and then drive the speed limit down the road.
You should slow down to a speed of 10 to 20 MPH or slow down as much as you need to so that you can safely make the turn.
When you do make the turn then you need to slow down enough to safely make the turn and then accelerate at the proper speed that the road allows you to drive at.