Consider: how much better than the line 'So you're fast' would 'I knew it' be @ https://www.youtube.com/watch?v=fIHH5-HVS9o
'So you're fast' is redundant dialogue because we've just seen that Barry Allen/The Flash is fast, so Bruce Wayne doesn't need to say it. The screenwriter shows and tells. Only showing was required. Telling as well slows everything down, and insults the intelligence of the audience at the same time. No dialogue at all from Bruce Wayne would have been good here. Just a reaction. Failing an informative silence and a reaction (that shows us something), if Bruce Wayne instead said 'I knew it' at this moment, that would tell us that he had heard of The Flash, that The Flash thus had a rep, and that Bruce Wayne had come to find out if it was true. This reputation would build The Flash character in our eyes. It would tell us that Bruce Wayne (with Batman's agenda) had spent time searching for The Flash; that would tell us he needed The Flash, which would tell us something about The Flash' power, and Batman's vulnerability. Instead, whoever locked the Justice League screenplay told us something we already know because we've just seen it. S/he has slowed things down. Spoon fed us.