Elsewhere he talks about how subsamples are chosen because they take but seconds to run instead of minutes. If a sample is small enough that you're saving that

Elsewhere he talks about how subsamples are chosen because they take but seconds to run instead of minutes. If a sample is small enough that you're saving that much time, and analysis is that cheap, you can still do five-fold or ten-fold cross validation in less time than the full data set analysis and get a very good idea on if your subsample is representative of the data or not.