I feel there is some misleading information here. This guy talks about the decay of a 12" as being slow. That just isn't the case. This is more audiophile gibberish. A larger cone will have more moving mass, but what makes it slow is the relation of the moving mass to the motor force (Bl^2/Re as a T/S formula). You can measure this, even though he frames it as some kind of intangible audiophile quality. Just look at group delay measurements.
But I don't even agree with his characterization of slow. For most people, it's going to be a matter of frequency response. He talks about a system being flat to the low 20s with a bunch of 6" drivers. A 6" driver would need to be extremely heavy to be flat to the 20s, or have very loose suspension combined with weak motor force. Or else he either had some kind of ported system or he EQ'd it to be flat to the 20s. The problem with that is both porting and equalization can add a lot of group delay.
I could go on with all the problems in this video, but I don't have time right now. Maybe later. Don't trust a word this guy says. NOTHING he is saying is backed up by any kind of research. It's all audiophile hot air.