there is a sweet spot between too slow and too fast, too slow will cause the engine to heat up, too fast will cause the block to heat up due to the coolant moving to fast to absorb the heat from the block giving you a false reading on the coolant while the block is over heating. The thermostat, even when fully open provides enough of a resistance to prevent the coolant from flowing too quickly. If you want the best fluid to cool the engine use water with some lubricant in it. Antifreeze is very poor at absorbing heat