I prefer a technique where algorithms and computer science come into play. e.g. dictionaries, hashes, binary trees, structured data based upon the actual uses, etc. Having a giant array of memory objects and then treating every bit of code like it's some edge condition is not appropriate.

I agree that Dictionaries and the like are under-used.

But I don't agree that LINQ is bad because someone on your team didn't know how to use a Dictionary and thought that iteration would be fast on large in-memory lists.

In many cases there are efficient LINQ constructs, e.g lazy lists, and First() not enumerating past the matching item. Hand-coding these is prone to doing it worse than Linq would. Teach your team how to use .ToDictionary() if need be.