Performance improved drastically (up to 50,000x) on several tests in bestbuy_4_analytics due to a change sometime between Nov 16th and 18th. Most likely causes are: SERVER-70617 and SERVER-70972.  Note that these are collection scan tests without any indexes defined at all.

The results have been reproduced several times in evergreen, as you can see in the trend charts here: https://spruce.mongodb.com/version/637bb59cc9ec4412eb36da6a/tasks?sorts=STATUS%3AASC%3BBASE_STATUS%3ADESC

Query MQL is listed here: https://github.com/10gen/workloads/blob/master/workloads/bestbuy_analytics.js