This article at NY Times was interesting. Especially the part about what happens to the speed by the time it reaches the other side.
http://www.nytimes.com/interactive/2015/07/10/sports/tennis/facing-a-150-mph-tennis-serve.html?ref=sports&_r=0
http://www.nytimes.com/interactive/2015/07/10/sports/tennis/facing-a-150-mph-tennis-serve.html?ref=sports&_r=0