Why Is MongoDB So Fast

MongoDB isn't like a traditional relational database. It's noSQL or document based, it provides weak consistency guarantees, and it doesn't have to guarantee consistency like SQL.

MongoDB is fast because its web scale!

Its a fun video and well worth everyone watching, but it does answer your question - that most of the noSQL engines like MongoDB are not robust and not resilient to crashes and other outages. This security is what they sacrifice to gain speed.

SQL has to do quite a lot, Mongo just has to drop bits onto disk (almost)