As someone who hasn't upgraded yet, what is causing the 15s wait after a locked system? That sounds terrible.
- check the logs. There may be an obvious hint there as to the cause
- quit all programs, menu bar items, launchd scripts, etc, and check whether the problem persists
- create a new user and see if that user has the problem, too
If you're a Linux user, or were a Linux user, check the logs, close programs, uncheck login items 1 by 1, run "launchctl list", etc to identify the specific cause.