Which statement is true about subsearches?

Which statement is true about subsearches?

A. Subsearches are faster than other types of searches.

B. Subsearches work best for joining two large result sets.
C. Subsearches run at the same time as their outer search.
D. Subsearches work best for small result sets.

Reference: https://community.splunk.com/t5/Archive/Looking-for-way-to-explain-why-subsearches-are-so-slow/m-p/479133