Nothing really included with PD16, if you bought one of the "Bundle" type options or periodically CL offers CD as a add on for minimal cost, this is the full CD version at the time. So, if you bought the "Ultimate Suite" product of PD16, you got CD6 as part of that package. However, if you bought "Ultimate" PD16 product, you got no CD.
The Essentials is a scaled back time bomb version for evaluation. However, since it does allow evaluation of a more recent product, one could say it's an upgrade for those features, but overall, it's an evaluation, really CL's new name for trial.
As far as making PD run slower generically, not that I know of. However, if you use some of the CD features, they can be very frame intensive and will slow down editing playback and producing final video, just like frame intensive features in PD like video stabilization.
Jeff