if a tachyon travels faster than the speed of light would an antitachyon travel slower?
A tachyon defines all FTL particles, not a specific particle.
If you were to define "anti-tachyons", they would simply be our garden-variety STL electrons, protons, neutrons etc.
I see your point; if the antiparticle of the electron (travelling slower than light on a timelike worldline) travels "backward in time" (according to Feynmann) i.e. FTL on a spacelike worldline, then it is a tachyon and its antiparticle is the timelike electron.
People keep telling us the "backward in time" is just notational; the difference between the electron wave function and the positron wave function in QED can be expressed by putting in t -> -t. So I don't think you can push this statement to draw conclusions from it.
The positron is visibly a real particle that leaves timelike tracks in detectors.
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