Anecdotal claims like 'not being good for music' or not being 'fast' are as unscientific as it gets.
The only thing that could be related to 'speed' is the group delay which is higher on the SVS but should still be inaudible.
The Arendal 1961 1s is a lot better in that regard and still half the price of the KEF.
The room, placement and room correction will have a significantly higher impact on sound than having a few milliseconds less group delay or a few percent less distortion.
I'm a KEF fanboy but from a consumer perspective I don't really see the point in getting any of those overpriced micro subs when you can get better performance and more features for significantly less money (and the same box size in the case of the SB1000).
The MIE subs looks interesting though if they can deliver similar performance to the competition.
I do sound for my HUGE church auditorium, we have subs that literally use (2) 21" Pro sound subs on each side of the stage. And an additional 2 more flying above the stage on each side. For a total of (8) 21" subs.
With only a couple hundred watts, it makes far more bass than ANY SVS sub out of their line up. You can FEEL the bass clear down into the 25hz range, at levels FAR above any SVS sub. The ENTIRE room is pressurized and you can FEEL the bass so strong that it is nearly scary. Not a fair comparison? Of course not, but..............
I think there is simply a usage for all subs, and not everyone NEEDS super loud and so on. The goal is not to get the loudest, but to use what one needs.