Yeap @_zorbash (at twitter) is still responsible for the API. It is true that we really enjoy our job at skroutz. :)

"Our MongoDB dataset is far bigger than our relational data, weighing in at 400 GB and constantly growing."

MongoDB keeps indexes in RAM IIRC. How much RAM does your server have? :-)

It runs on our most powerful server with 96G RAM :)

We have (at skroutz.gr) kibana 3 dashboards with 61,269,264 documents using the last month as time window and still no performance hit. Check our cluster resources in our blog post to get an idea.