tachyonic particle are merely hypothetical particle that always travels faster than light., But if real, how fast would they be?
This is like asking "how fast is a massive particle ?". The answer is "it depends". It depends on the reference frame / on the dynamics of the particle.
Depending on the reference frame of the observer, a tachyon could have any velocity that is faster than light. In some reference frame they propagate instantaneously. Different reference frames do not agree on if they propagate forward or backward in time.