One of my machines is an AMD Phenom.  I wanted to explore building
some new (to me) userspace, and this machine had a system which was
both recent enough (last month's LFS/BLFS) and with enough space to
make it seem like a good place to do it.

I finally got round to trying to build qt-5 (5.5.0), running
4.2.0-rc1.  For the minor packages before qt I had been building one
or two at a time, and leaving the machine.  I'm using MAKEFLAGS of
'-j4 -O'.  For qt I was a bit surprised to see that the system only
appeared to be using one CPU whenever I checked it (looking at the
taskbar cpu window in icewm), but I assumed that I was probably
looking at inopportune times (i.e. waiting for some dependency to
complete).  That build eventually failed in the qtwebengine part
(known problem with gcc-5.1, a patch for gcc is available), but only
after six and a quarter hours.

At that point I suspected that something might be wrong in my
scripts, but I eventually confirmed that my MAKEFLAGS were indeed
being picked up.  Then I fired up 'top' (procps-ng-3.3.10) and saw
that CPU usage was indeed maxing out at 100% (instead of the
expected 400%).

Back to 4.1.0, started the qt build - during the (long) configure
step the CPU total was between 100% and 115% (Xorg, 3 rxvt-unicode
terms, one vim session, one buildscript, and top) and then it
approached 400% as soon as 'make' started.

At that stage I assumed I must have somehow buggered up the 4.2
config - if everybody was hitting this sort of slowdown, somebody
would surely have noticed.  So, I downloaded 4.2.0-rc2 and built
that from 4.1.1, taking care to think about each configure option
(but nothing jumped out as likely to be relevant).  Booted that, but
the problem continues.

If I build _any_ package using MAKEFLAGS of '-j4 -O' it still only
gets the equivalent of one CPU.  Even a manual 'make -j4' (to ensure
it isn't a script problem). I haven't been building much since
I installed 4.2-rc1, not sure if the problem is specific to this
config / this system.

The machine is an AMD Phenom(tm) II X4 965 Processor and gcc is
5.1.0.  I'll attach my config, and also a diff from the good 4.1.0
config in case it is something obvious.  Meanwhile, I'll go back to
4.1 to contine my attempt to test the new userspace packages.  I
suppose that I can bisect it if I have to, but at the moment this is
not a good time.

ĸen
-- 
This one goes up to eleven!
(for this case, "goes up to one" would be better ;-)