I sometimes wonder if CS should be renamed Computer Pseudo-Science. I blame Knuth and (mostly) Dijkstra for propagating the falsehood that you can estimate the performance of real code on real hardware with a some elegant academic blackboard math which gives you the wrong answer for many practical applications.

It's not that Big O isn't useful - it's that it's taught as a set of "proofs" which somehow make it appear objective and "correct", when in reality performance is at least as dependent on cache architecture, median size-of-n, memory bandwidth, and other implementation details.

Anyone who graduates CS without having been taught this very forcefully - preferably during a practical project - should be refunded at least some of their course fees.