Grace Hopper famously used 30 cm pieces of wire as a teaching aid to show how far signals can travel in one nanosecond. Indeed, the speed of light has become a limitation for many computers. The Cray-1 supercomputer was built in a "C" shape to minimize delays and skew between signals. Modern computers minimize distances to avoid the effects of delays and skew.
Other physical factors have limited clock speed. Relay computers can switch only as fast as the contacts can physically move. Transistors (especially MOS) take time to switch. The growth of clock speed in microprocessors has slowed, due to the difficulty of dealing with the heat given off by faster circuits.
What limited the clock speeds of vacuum-tube based computers? Was it physical factors (distance, switching speed, heat dissipation) of the tubes themselves or their wiring? Or was the limit instead caused by items outside the processor, such as memory access time, input, and output?