Collect-then-iter round trip

collect::<Vec<_>>() allocates and copies; the next .iter() walks that copy. You paid for a heap allocation, a bulk write, and a second pass over data you already had in a stream — usually to satisfy a borrow shape that wasn't actually required.

Keep the iterator chain end-to-end and call the consuming method directly (sum, for_each, fold, min_by_key). If you need to share the values, take the iterator by reference (&mut iter) or restructure so the source is owned. Allocate only when the result genuinely needs to outlive the chain.