NGINX Not Serving "Fast" Stale Content with proxy_cache_background_update

We are running NGINX in front of our backend server.

We are attempting to enable the proxy_cache_background_update feature to allow NGINX to async updates to the cache and serve STALE content while it does this.

However, we are noticing that it still delivers STALE content slowly as if it's not serving from the cache. The time it takes after an item expires is very slow and clearly not served from cache - you can tell it's going to the backend server, getting an update and delivering it to the client.

Here is our configuration from NGINX:

proxy_cache_revalidate on;
proxy_ignore_headers Expires;
proxy_cache_background_update   on;

Our backend server is delivering the following headers:

HTTP/1.1 200 OK
Date: Thu, 28 Feb 2019 21:07:09 GMT
Server: Apache
Cache-Control: max-age=1800, stale-while-revalidate=604800
Content-Type: text/html; charset=UTF-8

When attempting an expired page fetch we do notice the following header:

X-Cache: STALE

However, when providing this response it is very slow as if it's contacted the backend server and done it in realtime.

NGINX version:

$ nginx -v
nginx version: nginx/1.15.9

Any suggestions, tips and config changes are greatly appreciated.

UPDATE

It seems that the nginx server is honoring serving stale content (as we have tested) but it also updates the cache from the backend on the same request/thread thus causing the slow response time to the client. I.e. it seems to be totally ignoring the proxy_cache_background_update on; directive and not updating in the background on a separate subrequest (async).