Currently, content is only compressed by the cache proxies for transmit to the end user. There is no compression between varnish and the MediaWiki servers.
While compression uses CPU cycles, it does improve response times by a huge margin, especially for cache proxies located outside the Benelux. It's worth evaluating compression for:
- Traffic between Varnish and MediaWiki
- Storing objects compressed in Varnish cache
And while we're at it: https://medium.com/oyotech/how-brotli-compression-gave-us-37-latency-improvement-14d41e50fee4 possible replace gzip with brotli for various resources?