[Development] Qt Compilation Speed

[Ulf Hermann]

> a) do we care

I care.

> b) what metrics should we use to define success?

I don't care whether it uses PCH or not. It should do the fastest thing 
by default. So, my metric is "time it takes to build <project>" on my 
machine. My favorite project is qtdeclarative, but I also build other 
things.

c, What are the trade offs? e.g. the std headers are probably used for 
something. Getting rid of them might involve avoiding the usage of std 
types in more places.

d, We used to build at least Qt itself with PCH by default. Why did we 
change the default?

best regards,
Ulf