What does timescale control in unity?
Are you ensuring vertical sync for your unity rendering? Does unity render at 60hz ? What may be happening is that the additional overhead of rendering the texture to syphon is pushing it over a 1/60th refresh, causing apparent dropped frames.
You may want to throw a timer in there and look at how long a frame takes to render with and without a client attached, and adjust the timescale programmatically to compensate?