Is heat the only thing that keeps CPUs from having a faster clock cycle or are there other limiting physical factors which keeps CPUs under 3~5GHz?
Slew rate is actually one of the more significant problems to faster processors, the tiny gate capacitence of a FET means that its output cannot switch instantly. Therefore, its output edges are rounded, try to go too much faster, and those round edges will start to give the wrong value, because they weren't able to turn on the next transistor in time. This is related to heat; (almost) all of the heat dissipation in a cpu takes place during that rise/fall, since charging or discharging a cap requires moving current, and while being turned on or off, the resistrance of a FET is high, though current is flowing. This means the power it dissipates is significant only during rise/fall. If you had infinite slew rate, then clock rates could increase, AND heating would decrease.