At the moment we only log that information from nginx's perspective, and there can be a lot of unaccounted time. It could be in nginx request queueing, or in tornado's (thumbor).
Related Gerrit Patches:
|operations/puppet : production||Whitelist new Thumbor-Request-Date header in Swift|
|operations/debs/python-thumbor-wikimedia : master||Upgrade to 1.12|
|mediawiki/vagrant : master||Add Thumbor-Request-Date to Swift header whitelist|
Actually doesn't show up for 200s yet, but I think that's because the swift proxy filtering kicks in only in this case (which is interesting in itself - error responses aren't subject to the header filtering).