The development of the Perseus, or at least the minimum design specs that would have been issued to cause it's development, are probably older than 18 months.
Now, everyone's complaining the Perseus is designed wrong. I agree, marginally, but would argue that Valkyrie is designed even more wrong.
The basic task of an interceptor is engage enemy bombers and destroy them before they can release their weapons. Only two requirements arise from this: speed and firepower. In the FSverse where many bombers also have a turret and/or Piranhas and Synaptics, and all bombers have escort fighters, some durabilty is also required to ensure you get your interceptor back.
Of these three requirements, the Valkyrie has only speed. It lacks firepower slightly in comparison to its contemporaries and it heavily lacks durabilty. Firepower is marginally excuseable since it's from the pre-shields age and things were more fragile then, but this only makes the Valkyrie's extremely weak hull even more of a problem. Both the Osiris and Amun have two turrets firing what are very powerful weapons in the pre-shield days and a Valkyrie attacking a defensive box of such ships is far too likely to be lost to their turrets, to say nothing of being intercepted in turn by escorting fighters.
The Perseus has some speed, but not as much as the Valkyrie. It does, however, offer firepower and durability at least no worse than its contemporaries, improving in two categories over the one it lost in.
The truth is that all FS interceptors, including every fan design I've ever seen with the exception of BlackWolf's Draco, are designed wrong. High manuverability should be the province of Space Superiority designs, who actually need it for their assigned role of dogfighting. Interceptors only need to outmanuver bombers; they should be fast, well-protected designs with average or above-average firepower and only enough manuverability to outfly their primary targets.
(If this sounds like it would obselete assault fighters, then it might if the interceptor and assaults aren't specialized into their roles enough, but as it stands, FS interceptors obselete space superiority fighters anyways because they're not specialized into their roles enough.)