To explain the parent comment, in a write heavy application, you would probably want to turn off transactions in Mongo in return for the performance gain. What

To explain the parent comment, in a write heavy application, you would probably want to turn off transactions in Mongo in return for the performance gain. What the comment you're responding to is saying is that when you aren't making many writes, you can afford to switch transactions on and have Mongo lock the whole database because the odds of having a read come in at the same moment are much lower than if you were making a write every second.