This is a simple example to illustrate the performance advantages of using Dictionary lookups over LINQ methods when doing multiple lookups on a large collection.
It's easy to forget the time complexity of LINQ methods. It's not uncommon to see LINQ methods like Single or First used within a loop to select items from a List. Depending on the circumstances, it may make sense to convert the List to a Dictionary and take advantage of the Dictionary's O(1) lookup time complexity.