Filtering performance: collections vs. metadata restrict
Summary: When designing a search solution you might consider using collections or meta data restricts to filter the set of pages. The question is which of the two options will result in better performance to filter search results.
Answer: The two solutions will perform equally fast. There should be no noticeable difference in terms of performance.
Still, collections are the recommended way to structure your index content into different logical sets and are particularly useful while navigating crawl diagnostics.