So, is it just lag, or...

Does Karthus' Q have a random interval of time before it pops off like Cho'Gath's Q used to? I feel like it waits patiently for someone to walk out of it half of the time and other times it just goes off instantly under someone's feet.

It could just be a feeling, or lag, or something, but it seems random like Cho used to be before they normalized the delay.