Exporting 12k documents is ok with mongochef took only few seconds, but importing these documents takes ages (many minutes). Importing bson file took 11 seconds with mongorestore. Mongochef should implement something like that.
I tried copy documents to different collection on same host, but this one is slow too. Maybe this is problem of number for concurent writers? Deal with hundrends of thousands documents or even millions is impossible with mongochef.