The only reason Dioscuri could go FTL is becuz of their authority. It has been already stated and proved that servants' core can't withstand going way above hypersonic speeds, though the exact number isn't known. There are two feats of Hakari reacting to lightning against Kashimo but I don't...
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It's literally stated they cannot go Faster Than Light for long periods of time. Dioscuri can go Lightspeed without any issues and Servants can react to that fine. I have no clue where he's getting "Servants Cores can't withstand going beyond Hypersonic" because every other casual Lightning Dodging/Light dodging feat says otherwise(not to mention it's
Saint Graphs that can't withstand the damage, not their Spirit Core). Otherwise a good bit of the thread is complaining about Lightspeed/FTL and then bringing in shit Japanese Translations... again.
Seriously, Fallacies long since stated that you can go with Subluminal/Superluminal or whatever and it's primarily up to the official translation and it goes with the same deal(Lightspeed/FTL)... which wouldn't matter anyway because we canonically got the lowest speed Servants can go... and that's Mach 1457 per second.
Like when the story tells you it point blank, it's time to just go home with this "Servants stuck at low supersonic-low hypersonic" nonsense.