"the remaining 2% were large batch requests", [which made up 50% of the work] .. who really watches that many shows on Netflix? What was in those batches, if someone is watching that much, why bother with serendipity at all? Most serendipitous thing you could do is shut off their subscription.
Note that they likely mean the list of candidates is large not the user history. This is for an API so perhaps 2% of client requests implemented batch requests, providing the opportunity for batch processing of that request.