I ran into this old image I think I created while uploading files into a system as fast as possible. x axis is the number of files already uploaded. y axis is how long it took to upload and ElasticSearch index.

Transient. Then bi-modal. One mode is very efficient. The other is like the data isn't indexed at all. In spite of usually being really fast; every once in a while, you get something that's terribly slow.