Well, Modul8 does not support native clients, so that means you are using QC Rehab and a Quartz Composer patch. This is really an unofficial way of getting Syphon into Modul8.
Does “Simple Client” show slow performance from the video sent from MaxMSP/Jitter?
Does the Syphon Server help file for Jitter show slow performance in “Simple Client”?
Unfortunately due to how GarageCube implemented Syphon in the latest Modul8 they chose not to expose “clients” (getting frames from outside of Modul8), only a single “Server” to send frames out (To Mad Mapper, etc).