Cat 01/02/2018 (Tue) 12:05:56 Id: 4e1f98 No. 503
Concurrency and performance.

Gridfs became unstable with JIT removing and reading at the same time, causing corrupted caches. Up to 1.8 that wasn't an issue since cached files would only be removed after a newer version was stored.

And the new implementation was able to serve 5x more requests.