build times

I have been investigating some build performance issues, and trying to
narrow down causes of slowness, and observed an odd effect, which was
suggested by a huge time difference between building from git and
building from a tarball.

If I do

make -C src/port all

and then wait 10 seconds or so and do

make -j 3

or even just plain

make

the build finishes much much faster (like 1m vs 5m) than if I had not
run the first command.

I have seen this on a Fedora VM (VirtualBox, W7 host, Athlon II X2) and
a ScientificLinux 6 machine (dual quad xeon E5620).

The setup is a vpath build configured thus:

../pgsql/configure --enable-cassert --enable-debug
--enable-integer-datetimes --with-perl --with-python --with-tcl
--with-krb5 --with-includes=/usr/include/et --with-openssl
--with-ldap --with-libxml --with-libxslt

Can anyone with a bit more make-fu than I have suggest why this should
be so? Can we tweak the make files so hacks like this aren't required to
get a fast build? Can anyone replicate this odd result?

cheers

andrew