For what it's worth, my fastest keyboards, in order:
Matias Tactile Pro 4, 120 WPM
IBM Model F 6110345, 119 WPM
Acer Aspire One D250, 119 WPM
You couldn't get three more different keyboards, but each has factors that make it fast.
Easy to distinguish keycaps are critical. The Matias and the IBM are best there, but the Acer wasn't particularly bad (unlike some of Acer's later models) in that regard.
Short distance to actuation helps, but as evidenced by the Model F being in there, it's not critical.
Short travel can also help, because it means you can recover from bottoming out more easily.
Force needs to not be too low, or you'll be prone to errors (part of why I don't get along with Cherry blues, there's other precision and feel concerns I have), but it also can't be too high - this is why a Model M isn't among my fastest.
Preload, however, must be low, unless actuation is extremely early. Another reason why the Model F works for this despite having such long travel and late actuation.
Key spacing is important, but this one is widely variable based on the operator's hand size. The Acer has 16.95 mm key pitch, as opposed to the 19.05 of a standard keyboard. I found that size to be faster, but if my hands were any bigger I'd have found it cramped.
So, a mechanical keyboard may not make you faster. Before I got the Matias board, my fastest boards were an atrocious netbook keyboard and one of the greatest IBM aircraft carriers ever produced. But, it was a far better typing experience.