It was not that the arrow was too fast for Lex to detect, but rather that Cornelius didn't use a normal arrow to begin with. So far, whenever Lex fought another immortal he had the advantage because they would try to use their control over laws to influence Lex. However, owing to the fact that he had a Dao level ingredient in his body, among other things, affecting the laws of his body was easier said than done.
He in turn, could affect his opponents however he wanted, or even rely on his many other abilities to gain the advantage. But Cornelius launched an arrow targeting not him, but laws. Those laws did not try to change anything about him. Instead, he just felt the law of physical force smash his chest with the strength of a missile large enough to wipe out a city.
In essence, the laws were not changing anything about him. Instead, they were directly acting on him. The distinction was minor, but the point was that it worked.
