Pre-compose or pre-render your clip.
You have posterize time on all layers. You only want to posterize time on the layer with the echo effect applied. You will need to track the back wall and attach that tracking info to the middle layer to keep to keep the frozen frames following the camera movement.
The theory behind the effect is that you posterize time on a layer so that see multiple copies of the same frame then you set up echo to repeat those repeated frames. You put your background footage beneath the racket layer playing in real time and you put a copy of your racket layer above the footage playing in real time so that the repeated frames are beneath the real time footage.
If you carefully look at my screenshot you'll see that posterize time is only applied to the middle layer.
Thank you guys, finally got a result over the weekend!
The idea with the pre-compose from mylenium was good, but Mr. Gerard delivered the core solution: Tracking the background.
The whole project got more difficult because of the bad resolution of the high speed camera, I will focus on that for the next video.
Thank you for your help,