Higher flow always means more cooling, the only tradeoff is the water usage - you will never get BETTER chilling power by turning down the flow - better efficiency perhaps, but not faster chilling. Just like these other guys, I cannot stress enough the importance of stirring. It can make the difference between chilling in 45 mins vs. 15 minutes.
If you want a dramatic example, while chilling, once the outlet water from the chiller has grown cool (but the wort is not done chilling), give the wort a stir while feeling the outlet water - it'll immediately heat up.
You'll make a huge difference if you stir it every 5-10 minutes at least, so the wort is always at least slowly moving - and even more difference if you can keep it stirred constantly (though usually this is done by a recirculating pump or motorized stirrer, rather than by hand)