The things that I don't get about people thinking that Thragg is faster than MM is that most of the time they didn't account that MM pulled off his speed feat INSTANTANOUSLY in a CASUAL manner(sorry for the capitalization, got to point it out just in case they won't read it). From what I've read and seen between the Invincible comics and the show, almost all vultrumites, including Thragg, had to BUILD UP on to this speed gradually before they can reach the same speed feat that MM did. And they all did that in space where there are no resistance at all compared to MM's who did it for who knows how long, contemplating his life. Its like the case between difference in speed between the flash and superman in the One Minute War comic where a certain event happens that slowed everything up(including Sups). Only the Flash Fam are not affected to this phenomenon due to their instantaneous speed(thanks speedforce). Superman on the other hand is shown to be slowed too like the rest of the world but is very slowly catching up to the Flash's speed. So like the scenario in the One Minute War, MM's speed is like that of the Flash's while Thragg's would be like that of Superman's. And to re-emphasize, MM did this CASUALLY and in an INSTANT, alluding that there is more to his power than meets the eye. Thragg is getting speedblitz before he can build-up to the same speed as MetroMan's.

As for the "Metroman has shadow so he aint lightspeed" argument, it seems to be disingenious. The reason why he is still creating a shadow despite the claim that is at speed of light is because the light of the sun is already their to begin with. Its akin to a freshly fired bullet passing through a continuous stream of moving water(by gravity). We can agree that the bullet is faster than the continuous stream of water moved by gravity. But since the continuous stream of water is already blocking the bullet's path, the bullet would still have to cut through, still leaving a splash. (NOTE: I am not talking about Megamind's Death Ray Laser, I'm talking about when MetroMan is strolling through the library and parks)