This issue has been resolved and cache purge performance has returned to normal.
Posted 14 days ago. Sep 07, 2017 - 10:41 UTC
Cloudflare is experiencing delays processing single-file cache purges via the API. The "Always online" feature is also affected. We are actively working to eliminate these delays. In the meantime, you may use the "Purge All" feature, but note that this may cause a surge in traffic to your origin site.