Still a false statement...totally sub, enclosure, and install dependent.
Saying that is an 18 inch sub is "slower" than a 10 inch sub is a myth.
Done correctly an 18 inch speaker will be just as fast/quick/transient as a 10 inch speaker.
My last setup of a single 18 ported was tighter and quicker than a lot of 12 inch setups I've heard.
Read up people...learn something.
http://stereointegrity.com/docs/WooferSpeed.pdf