I tried uploading a cohort from a recent A/B test (1,780 unique user_id’s). The async validation took about 5 minutes to complete.

If I create a temporary table with the data in my CSV and run a join with the user table against a slave, the query to validate that these users exist takes about 400ms if I use user_id (primary key in enwiki.user) and about 3s using user_name (unique in enwiki.user). 

What’s the reason why it takes so long to validate a cohort in the application?