MongoDB aggregate performance compared to Postgres
One difference to consider is the amount of IO happening. For Postgres data is normalised into tables and tables are read one page at at time. To complete the SQL it need only read however much disk is required to store all of table ppd_2015_perf.

Mongo's unit of IO, however, is a whole document.  All the denormalised/ duplicated stuff comes in along with the three fields in which you're interested.

Of course there a lot of things assumed here - RAM, compression, disk performance and data models being some.