With no password in key: mildly cleaner to drop entries on password change, even if the cache didn't get the command to drop the key, the next login would overr

With no password in key: mildly cleaner to drop entries on password change, even if the cache didn't get the command to drop the key, the next login would override the old key's value anyhow, instead of potentially a key per password that was valid in the short period around a password change Of course, if you have any validness of old sessions / passwords around a password change, you are doing something wrong. My personal wondering is, considering KDF is meant to be expensive, why is IO more expensive to the point it needs a cache?