I'll answer with what I do when I pitch on top of a yeast cake. I chill in the kettle, and rack directly on top of the yeast active yeast cake. Then I use a paint mixer to areate which fully stirs up the yeast cake into the wort. When I do this I typically have very active fermentation within 45 minutes. Occasionally this can be too violent and I blow the lid right off my fermenter, but that doesn't always happen.