Yet nonetheless we see that the giant splash happened in the space of a few panels, and regular civilians were able to see the leftover waves coming, discuss it and attempt to move to higher ground. It does seem like the waves were moving much slower after the initial splash.
Also, the Psykorochi vs. Tatsumaki fight is taking place at accelerated speeds (faster than ordinary human perception) so what feels like a long time from their perspective might only be a few seconds from our perspective. For example, the entire moment where Tatsumaki took the Monster Association from the ground, empowered it with her energy, tossed it down and drilled it into the earth took place while Darkshine, Atomic and Puri Prisoner were still mid-air after landing their attacks (entire sequence was only a few seconds in other words). This is why I believe it took "so long" for the waves to hit, they were still coming on quickly, just not that quick from the fight perspective.