We have no plans on implementing local/push notifications on WP8 in the near future. I recommend that you do this youself natively for now.
Our timer implementation is as good as it’s going to get on WP8. We’ve fine tuned it as much as we could. If you have very very short millisecond intervals, like less than 16 milliseconds, then you’re going to notice some random lateness problem which is really coming from the operating system.
That said, you might have other performance hits in your app that is causing these delays. For example, display.newText() is an incredibly slow operation on WP8 and we can’t make it any faster on our end because the native OS APIs which generate the text bitmaps are the cause. If you’re updating text on the screen regularly, then that’s the cause of your performance issues. You’ll need to switch to bitmap fonts instead or cache the text on startup to be re-used when needed.