It depends on how seriously we take light speed in One Piece.
Zoro dodged Kuma's paw pads which Kuma said he launches them at light speed (though this doesn't mean they necessarily maintain light speed for the entire attack?) and post-TS Luffy using CoO said light speed seemed slow to him (one of the dumbest quotes in Manga history when placed in context to future feats lol).
If we take light speed in OP and as literal light speed, then Zoro absolutely fodderizes Flashy. Reddit calced Flashy's maximum (shown) speed at only 7% of light speed, and if that's the case then Zoro utterly crushes him with superior reaction seed and massively superior AP.
Hype statements have put Flashy as FTL but that's probably bullshit, and if we're just going by what Zoro has actually shown speed-wise post TS, then Flashy puts him down fairly quickly.
So there's a couple of ways of looking at this matchup, depending on how you interpret Oda's literal non-understanding of how light speed actually works. If Light speed actually is slow to characters like Post TS Luffy, then every single post TS opponent Luffy has come across should've been utterly negged speed wise as none of them have shown anything even approaching light speed.
TL;DR Oda is a moron that makes this matchup open to interpretation.