it's been well known (i'd heard it numerous times) that the maximum clock speed of x86 is somewhere <6GHZ. as in "you can make a 10ghz x86, but it would spend half the time idle". Bursting to 5.6ghz (or 5.8 iirc) is possible, but there are constraints that are physical in nature that prevent anything faster.
Once single core single threaded CPUs hit ~4GHz the new "frontier" was the core 2 duo, then the core 2 quad, and now we have desktop chips with 16c/32t (and beyond).
Once single core single threaded CPUs hit ~4GHz the new "frontier" was the core 2 duo, then the core 2 quad, and now we have desktop chips with 16c/32t (and beyond).