keystone.conf [cache] enabled is not enabled.

Caching greatly improves the performance of keystone, and it is highly recommended that you enable it.[source]

Caching is not completely configured.

Although caching is enabled in keystone.conf [cache] enabled, the default backend is still set to the no-op backend. Instead, configure keystone to point to a real caching backend like memcached.[source]

Memcached isn’t reachable.

Caching is enabled and the keystone.conf [cache] backend option is configured but one or more Memcached servers are not reachable or marked as dead. Please ensure keystone.conf [cache] memcache_servers is configured properly.