Can one database affect the performance of another in MongoDB?
I can add, that WiredTiger and mongo 3.2.11 will not help you. Theoretically it hasn't global locks, but in practice I have a bit more beefier server (12 core, 96GB RAM) and two databases. DB1 is in production (it is only 300GB in size, so mostly in RAM) and DB2 which I prepare for next release. 

Surprisingly, found that mass updates and deletes on DB2 (also 300GB), even in one thread (one console or java application) make regular indexed queries on DB1 run for 5-10 SECONDS. 

When I stop all activity on DB2 it returns to 3-400ms.

In other topics I've read that deleting by unindexed field might lead to such behaviour, but unfortunately, not in my case. I update/delete by "_id" and still performance is terrible.