Ivy opt-in preview should warn about long compile times


# 🚀 feature request


### Description
I follow the instructions for opting into the Ivy preview, and then run `ng serve` and see "0% Compiling" for 20+ seconds on my machine.


### Describe the solution you'd like
When we need to do a long `ngcc` style compilation, we should consider warning users or tell them what we are doing.