Comment by neonsunset

LINQ is fine (but enterprise style never is, yes), it’s a matter of scale and what kind of a domain the code is targeted too. C# needs to be approached a little like C++ and Rust in this regard. Having standard performance optimization knowledge helps greatly.

Also can recommend reading all the performance improvements blog posts by Stephen Toub as well as learning to understand disassembly at a basic level which .NET offers a few convenient tools to get access to.