Among the many difficulties with a JLA movie is that "super-speed" is a completely broken concept that has never, ever been used consistently. It's easier to get away with it in comic books, but I don't think you can make it work in a movie.
Flash should beat any villain without super-speed before the villain knows what happened, even if the villain surprises him (
this scene from the JLA cartoon where Flash stares at a giant boomerang for an entire second before just letting it hit him is typical of how hard it is to portray his speed consistently). And any fight between Flash and a villain with super-speed should be nothing but a blur to everyone else. Either the audience sees nothing but a blur or the whole thing needs to look like a bullet-time scene with everything & everyone frozen in place while Flash fights his enemy. I doubt you can make a watchable movie in which every action scene is either a blur or a guy moving at normal speed through a frozen environment.
All this should be true of Superman (and the Martian Manhunter) as well. It's a little easier to ignore Superman's speed if he's on his own, but if he's sharing the screen with the Flash, it's much harder. Even Wonder Woman (who can move fast enough to deflect each individual bullet from a machine gun volley) should be mostly a blur to a normal human being (including Batman).