Given what it is, it would be as quick or quicker than a needle on vinyl. I.E. The latency would be exactly what the specs for the hardware would be. Or to be more clear, Its not the device that creates the tone that introduces latency. but the hardware that routes it.
Actually - there will be an amount of latency introduced from the simple fact that it is coming from the iPhone hardware. That is, there is a latency that we have no control over, which is latency between touching the iPhone's surface and the Tonetable software receiving that touch message, and also within the iPhone's audio output.
I think this demo video is a good example of scratching using Tonetable: http://www.youtube.com/watch?v=MKYdmcU031I
Oh, ok.. well that makes sense.. if there is a latency introduced though it seems to be negligible.. It's feels quicker than a needle on a record.. I've never seen these types of latencys to ever be a problem no matter what system you are running. Most cards can handle 256ms which is completely acceptable for scratching.
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I think this demo video is a good example of scratching using Tonetable: http://www.youtube.com/watch?v=MKYdmcU031I