Frame loss capture tool causing frame loss?

You should probably use the performance hud for validation instead, as it doesn't add any extra instrumentation and just looks at the compositor statistics. The frame loss capture tool is more for diagnosing frame losses you already know about.
The hud also has a nice frame drop indicator mode for just showing an "F" in the corner of the screen whenever a frame is dropped, so you can identify areas that need a closer look without having a bunch of stats in your face all the time.