Since increasing speed moves you away from c in FTL, there is no upper limit to tachyonic speeds, although there are practical limits, given the minimal level of energy needed to maintain particle integrity (remember, less energy = more speed in superluminal space). And while rest mass in subluminal space is real, positive, and increases due to special relativity as v approaches c; in luminal space, rest mass is zero and v always = c; and in superluminal space, rest mass is imaginary at v=c, but becomes real, positive, and decreases when moving faster than c.

An implication of this is the reversal of time dilation effects with regard to acceleration. In both STL and FTL, as one approaches c, one ages slower with regard to the larger universe. That is, the universe will age far faster than a spaceship barreling along at 99% of c. However, in FTL, approaching c means slowing down. If, instead, one speeds up, traveling at ever higher multiples of c, you would age faster and faster compared to the rest of the universe. This, of course, would be a major disadvantage of FTL travel if ships weren’t encased in a Markov Bubble when superluminal (more on this later).

As one can see in the graph, it is possible to have a velocity of 0 in subluminal space. What does this mean when motion is relative? That you are at rest with regard to whatever reference point you choose, whether that be an outside observer or the destination you wish to travel to. A velocity of 0 in subluminal space translates to around 1.7c in superluminal space. Fast, but still slower than the velocities of many FTL particles. Indeed, even low-end Markov Drives are capable of 51.1c. Nevertheless, if you need to reach a destination as quickly as possible, it can be worth the delta-v to bring your spaceship to a complete stop with regard to your destination before transitioning to FTL in order to get that extra 1.7c of velocity.

Were it possible to directly convert subluminal mass into superluminal mass, without a Markov Bubble, 1.7c would be the highest possible speed achievable, as there is no practical way to further accelerate the mass (i.e. further reduce the energy state of said mass) aside from chilling it. One can’t suck propellent into your tanks, for example. This would be the second major disadvantage of FTL travel, again, if not for the use of a Markov Bubble.

The third disadvantage would be the fact that matter in superluminal space behaves radically differently than in subluminal space, to the point where life as we know it would be impossible to sustain. This, again, is circumvented via a Markov Bubble.