CURRENT: Compile time of ports has increased significantly since LLVM 4.0

We build pretty often world and ports.

With poudriere as port building backend, The daily/weekly builds have now grown
in compile time significantly since approx. the introduction of LLVM 4.0 into
FreeBSD.

The build box is a LGA1150 IvyBridge XEON system with 3,4 GHz and 16 GB RAM.
The configuration of poudriere (most recent ports tree version, as well as most
recent CURRENT host and most recent CURRENT jail:  FreeBSD 12.0-CURRENT #4
r316067: Tue Mar 28 06:26:18 CEST 2017 amd64).

Within the past 4 - 6 months, wekbkit-gtk2 and webkit-gtk3 took ~ 1h 90 min,
or, when started together, ~ 2h 40 min. Now they take each(!, even separately
started and indivuidually conmpiled without concurrent jobs) more than 3 hours.

The same with /editors/libreoffice. On this bos it took 2h 40 min to compile
libreoffice 5.2.X up to 5.2.6, now it graces or is beyond 4 hours!

This observation is taken from a time span of 5 - 6 months and even with worst
case scenarios like parallel start of webkit[2]-gtk[23] in parallel.

So, does someone also diagnose this compile time stretching? I also see this
while regularily building ports via "make" in /usr/ports.


Thanks in advance,

Oliver