How much faster of a sub is the REL T5X vs SVS SB12-NSD

"fast" is a totally inappropriate term to use in regards to subwoofers. The speed at which the cone moves is determined by the frequency it is reproducing, the higher the faster. If a driver can't keep up with the frequency it rolls off. Most subs break-up before they roll off. Larger drivers or multiple drivers are always better because they produce more output with shorter excursions = lower distortion. 

A better term would be "dynamic" Some subs are more dynamic than others. This is very much dependent on the size of the driver and the amplifier used to drive them and the plate amps used in subwoofers generally stink not to mention a sub enclosure is not a great place for anything especially if it is a sealed sub due to temperature control. Amps working hard get hot so you can only use amplifier types that do not generate much heat. Passive subs with outboard amplifiers is a much more "dynamic" approach.  

This business about smaller driver being "faster" is totally and completely wrong. It is another example of lay intuition run amok. 

I have recently installed 2 REL T5/X into my system. I have been trying to integrate Mid/Base and Base response with my Wilson Benesch Square 2 Speakers as they are known for a fast base response. I wanted some additional base fill and soundstage improvements provided by a subwoofer. I have tried the SVS SB 12 sub and was not pleased with the results, just wound up with a muddy base response no matter what adjustments I tried using the app. My ear leanings are to higher frequency base so the big sub seem to sacrifice some speed that for more lower frequency. I was quite pleasantly surprised when I installed the 2 REL T5/X subs, first they were easily set up, and after some reading and trial and error on location, dialing in the frequency and volume, they were up and running. I am very pleased with their integration with with the Wilson Benesch Tactic Drivers, and while still fine tuning the subs somewhat I think I have found my solution.

I just purchased a pair of REL T5x and like @Pooch2, I was very pleasantly surprised at how easy they are to set up and dial in.. The REL’s work wonderfully with either my upgraded/modded MMG’s or Sonus Faber monitors. Integration with either the little Maggies or the SF’s is seamless. I’ve had a number of subs before these, including REL T5, but the T5x’s are by far the best in my experience.

I followed REL's setup suggestions closely and am thrilled with the results.  

Sure, this is just one guy’s experience, but I do hope it helps some of you other guys and gals.

The notion of a "fast" subwoofer is entirely inappropriate. The speed a subwoofer can vibrate at without distortion is it's frequency response at a given volume. A smaller driver is at a distinct disadvantage because it has to travel farther (faster) to produce a given low note at a given volume than a larger driver. As a rule, larger drivers have less distortion than smaller ones and do not have to travel a far (faster) to produce a bass note at a given volume. Producing 18 Hz with authority requires at least four 12" or two 15" drivers in your average listening room. In a 16 X 30 foot room I use eight 12" drivers. This is a two channel system.