Some discussion is at https://groups.google.com/forum/#!topic/mongodb-dev/_1IrogzovEQ. When I create an index with background:false then many (all?) operations in the db are blocked even for engines like WiredTiger that don't require a per-db writer lock. The URL above shows thread stacks where background jobs (TTLMonitor, ClientCursorMonitor) get blocked on a per-db lock by a background:false index create. I assume bad things can happen when TTL enforcement doesn't run for too long.

This creates other problems as "show collections", db.$foo.getIndexes() and queries from other collections in the same database will be blocked for the duration of the index create.

While background:true is the workaround background index creation can take more time.