A large collection is a collection that contains enough values that the cost of processing it becomes a cause for concern. Depending on the application—where it runs, who uses it, and how the collections are processed—a large collection could be anywhere from 500 to 10,000+ values.

Generally speaking, performing any kind of operation on a large collection only once isn't a big deal. The problem with this premise is that rarely do applications process collections only once. Often, the same collection is processed several times within the same function.

With Immutable.js collections, the cost of processing large collections is especially important because, since these collections are immutable, you're constantly creating and processing them.