src/mongo/db/commands/mr.cpp:1135:                LOG(1) << "  MR - did reduceInMemory: size=" << oldSize << " dups=" << _dupCount << " newSize=" << _size << " time=" << t.millis() << "ms" << endl

On log level 1, this message prints thousands of lines to the log file, even for short map/reduce jobs (10000 lines in 20 seconds). This makes it hard to diagnose other issues unrelated to map/reduce tasks on that log level.

I'd suggest to increase the level to 2 or 3.