Our production instance is proxied by nginx, which also caches all cacheable responses. Nginx is configured to log 'cache status' for each request – whether it was served from cache (HIT) or from upstream (MISS or EXPIRED). EXPIRED means that response was found in the cache but its TTL had expired.
Since the beginning of this year, there were 300940 requests to /avatar. Breakdown by cache status:
Note the share of EXPIRED requests. CPU time taken to process them could have been spent in other ways.
Please raise the max-age for avatars or make it configurable.