internment | Interning of data in rust

Each method in stream API does not create a new loop... it combines methods and executes them in single loop once it hits termination method (like collect). From the documentation:

Stream operations are divided into intermediate and terminal operations, and are combined to form stream pipelines. A stream pipeline consists of a source (such as a Collection, an array, a generator function, or an I/O channel); followed by zero or more intermediate operations such as Stream.filter or Stream.map; and a terminal operation such as Stream.forEach or Stream.reduce.

Processing streams lazily allows for significant efficiencies; in a pipeline such as the filter-map-sum example above, filtering, mapping, and summing can be fused into a single pass on the data, with minimal intermediate state. Laziness also allows avoiding examining all the data when it is not necessary; for operations such as "find the first string longer than 1000 characters", it is only necessary to examine just enough strings to find one that has the desired characteristics without examining all of the strings available from the source. (This behavior becomes even more important when the input stream is infinite and not merely large.)

The only method that creates new loop is flatMap.

Other than that, yes streams are slower, but not by much. They are far easier to read, write, modify though. And to add to that, they are encourage immutability, which is a trade-off of performance for less buggy code.