compile time regressions (was: merging for 3.4)

[Neil Booth]

Mike Stump wrote:-

> No, no, you have it backwards...  PCH is going to expose all that slow 
> code that should be fast, that has to be fast, so that we can fix it.  
> By being 12x faster, or just 2x faster, there is _much_ less room to be 
> slow, not more, really.

But slow code in things that PCH elides won't get exposed by the very
nature of PCH.  e.g. drastic CPP slowdowns (not gonna happen!), or say
the parser.

Neil.