"Why not iterate a giant array instead of using hashes? Why not use LINQ everywhere, repeatedly and inefficiently filtering a massively excessive superset of the needed data?"

There's not prematurely optimizing and there's criminal neglect.

Sure, if you take the First Year Student™ approach to problem solving then a simple database query and processing might be O(scary) and fail to perform.

Generally speaking though, a few milliseconds away from that clever bitwise operators hack would be fine.