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