Thanks for the reply.
I have only one instance of the script Syphon.
Each camera has an instance of SyphonserverTextureCustomResolution and SyphonServerAlphaOne.
I need all the cameras active all the time because they are projected on different screens.
The problem might also come from Unity itself by using multiple cameras in a scene… I tried to optimize by using layers occuling but that part is still taking ressources.