First of all let's remember what the purpose of water cooling is: to move the heat from the hot parts of the computer to the outside of the case. We cannot destroy the heat, we can only move it. How is this accomplished? First we use a small amount of water to absorb the heat produced by the cpu in the cpu block. Then we move this quantum of water to the radiator where the heat is transferred through the fins of the radiator into the air. This cools the water back down. Now we send this same quantum of less hot water back to the cpu block to absorb more heat. And we do this over and over again. The faster we do it, the better. The more times per minute that you heat up the quantum of water and cool it off again the more cooling capacity your loop will have. Just like the fact that the faster the air is passing over the radiator the more heat will be transferred to the air. This situation is not analogous to trains, unless you have the trains loading and unloading passengers. Now the total passengers you load and unload over a full day, is a more analogous, if still inapt, to water cooling. Further the voltage in a light bulb is analogous to the head pressure of the pump, not the flow rate, which is analogous to the current in the wires. And finally, radiators do more work when there is more flow, and do less work when there is less flow. You would never want to slow the water down in a radiator, this would never provide you any benefit (in terms of cooling).