Speedsters in Champions should generally not be done by simply buying incredible levels of SPD. SPD should be seen as how many different things the character does. Punching the same villain five times is not necessarily five actions - it is better represented as one action, but the punch has the Autofire advantage. Punching five different minor crooks is using the Area Effect advantage. Or either one can be a Multiple Move By, still one action. To build a speedster, buy only a slightly higher SPD, but lots of special effects on the actions.

The main thing that makes SPD greater than 12 prohibitive is that the SPD 12 character's player gets too much of the GM's, and the group's, time, relative to the other players in the group - the point cost is secondary.

It is like the Flash, as you mentioned, but in a Justice League of America comic book. He doesn't get that many more comic frames on each page than, say, Batman, even though he is in theory acting far faster. He just theoretically does more things per frame.

Page space and play time are very similar in this respect. They are each indicative as a measure of how much of the comic reader's or the game players' attention the character gets. Just as you wouldn't want the Flash to fill an unproportional fraction of the pages of the JLA book, in the JLA role playing game, you wouldn't want the Flash's player filling all the play time with his character's actions.