Not all, but most of his action scenes in this movie were unwatchable to me due to his ridiculously unnecessary blinding and seizure inducing lightning effects. I don't have epilepsy but I've definitely gotten more sensitive to stuff like that with age and I had to either look away or close my eyes during most of his parts. Great movie but I don't know what Snyder was thinking making it so dramatic.
Part of the problem, I think, is that it is difficult to show fast movement in a convincing matter. Slow motion tends to look fake. The rapid movement of the limbs can look cartoonish. Some series with fast moving characters have defaulted to making the runner essentially invisible (see a couple of episodes of Charmed for this option).
JLA opted for the slow motion option when doing closeups and added the lightning effects to mitigate the slow motion; it also played into him generating power for Cyborg to split the mother boxes.
I don't think there is any good option to show a speedster. The Flash TV show did a relatively good job, though I haven't watched it in several years. I'm sure the movie producers wanted their flash to be distinct from the TV flash, both in personality and visual effects.
I was watching a stoat in a field once trying to make up it's mind about crossing a gap. As it darted back and forth it was too fast to see, it just appeared at each end of its little run. So having a superhero be invisible at speed makes a lot of sense to me.
Not a fan of the actor in the role. Adam Brody would been perfect as Barry Allen 'the Flash' ten years ago, still be perfect today actually, wish he had the role instead.