Here is a scan of him working on an alien mind and it is taking time for him to read it and communicate.[/B]
As soon as Storm asked him to form a mind link it was practically instant though , he was able to tell us the Dragons thoughts immediately. He then was going to probe deeper but he sensed that the Dragons was upset that he touched it's mind and for some reason stood he stood in one place while it threw a building at him. Off-topic but Usually telepaths would get react to sensing that they're about to be attacked.
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http://img511.imageshack.us/my.php?image=wolverinevol325image13jd6.jpg
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You can put a speed on the conception of the thought...which is why everyone agrees that Superman would speedblitz Xavier. But I don't think you can put a speed on telepathy, as what you're suggesting would mean that it travels many millions of times the speed of light.
Xavier has accessed the Universal Collective Conscious(Which I don't even think exists physically) with his telepathy, Nate and Jean have communicated through dimensions...seems more like a tranverse space/time sort of thing.
And then psionic energy has differed in all it's forms: Psionic energy has been called TK Energy and TK works at the speed of light.(I have a scan)....but Psionic energy has also been called life energy and telepathic energy...which makes the whole thing convoluted and headwrecking.