Once the Falcon 9 Full Thrust 1st Stage reaches "MECO" - main engine cut off - and separates from the 2nd Stage how much velocity does it lose? Specifically at "MECO" its velocity is approx. 5,900 km/hr Does the 2nd Stage separation "push off" from the 1st Stage significantly slow down the 1st Stage before the 1st Stage engines ignite for the boost back burn?
No, only a very small amount of speed is lost. I'm not sure exactly what it is, but no more then a few miles/hour. Otherwise you would see the two separate much faster.