The speed limitations on CPUs are due to transistors not being able to switch that swiftly due to the capacitances inherent to a MOSFET. You also have to make sure that your data has made it to its intended target before you try to read ti from there, hence why higher speeds can lead to stability issues, smaller transistors lead to higher speeds as you have smaller capacitances and less physical latency of the signal hence why some sandy bridge machines can hit 5.6GHz, but you wont see intel selling a CPU clocked that high because it will be producing a ton of heat and require a massive heatsink to keep in check
Now, the reasoning for the phone "operating" at 5.4GHz, it doesnt operate at that level, all of the signal processing and data processing is done at 1MHz or less, once all the processing is done its strapped onto a 5.4GHz carrier wave, when it gets to the end it gets extracted from the carrier wave, but no calculations or processing is occurring anywhere near the transmission frequency, its just a lot easier to transmit stuff at higher frequencies