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

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.

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

"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.