Best way to keep large ports uptodate

[Rob Hancock]

Hello everyone,

I'm a recent convert from debian based systems.  I must say that I am
very happy with freebsd so far.

Currently I've been using portupgrade to keep all my ports current, and
it works great.  With very large ports such as KDE/base/libs/etc it takes
several hours to compile on my little 600MHz laptop.  Is there a way to
keep the previously compiled objects around so when I upgrade the
next time only the changes have to be recompiled?  Or am I better off
just waiting until major updates and doing a clean compile then?

Also, top isn't showing any percentages as far as user, cpu, nice etc.
I remember reading that it is usually caused by world/kernel not being
compiled and installed together.  However I did just that...rebuilt world,
kernel and installed per the handbook.  This is with 4.10p2.  Only thing
I can think of is that I have CPUTYPE as I686 in my custom kernel and
as p3 in /etc/make.conf.  Should I try changing /etc/make.conf to I686?
The computer is a 600MHz p3.

-Rob