I have a rule about trying not to take part in Dragonball threads any more but I will just point this out.
This is not entirely accurate.
In the original webcomic, Saitama never really comments on Flashy's top speed directly. We can argue that given how Garou dealt with Flashy versus how he failed to deal with Saitama suggests a significant gap but it's hard to say exactly what that gap would be (parrying everything easily vs. parrying a lot of stuff and eventually being overwhelmed)
In the redrawn manga, Flashy has not shown his absolute best to Saitama at time of writing and, when explicitly not going 100%, Saitama commented that Flashy was "pretty fast,"
When I did the speed algorithm for OPM generally, I concluded that Flashy is in the "small but significant fraction of lightspeed," I figure he's single digits, likely 1-2% if you were to press me for an exact number.
Obviously, there's latitude to this depending on how the redrawn manga handles things with Garou and Saitama going forward but I would be very leery about saying things like "high fraction of lightspeed," without further evidence coming from the redrawn manga. We can say Saitama is "faster than Flashy," and we can say "some significant fraction of lightspeed," but it's hard to say more than that without more feats.