Yeah, the Debugger sucks up a bunch of system resources.
make a text object and then use a Set text "fps" to make your own FPS meter, and pulse it every 60*dt. This will be more accurate, but remember that the FPS meter you just made sucks up a little bit too.
system=>every 60*dt-----------TextObject.Set text "fps"
There are a couple of others you can use to help optimize, like getting the objects "count" and setting it to the text. I think there is even a CPU and GPU expression, but I really would not trust those due to hardware acceleration, and crappy web browser design. In Unity I have had some major heart breaks thinking my overhead was cool, but the meter i was reading was designed for CPU rendering only... It ignores the GPU completely. C2 might be the same way.