Normal CPU usage with a PPP connection

[Dylan Smith]

Hey guys, i'm running 6.3-R with (i think) userland ppp to connect to my
ADSL provider over PPPoE.
I build a number of graphs(mrtg) for system stats and i am noticing that
under heavy load on my ppp connection, that is sustained 150KB/s for an
hour or so my cpu usage, based on my graph, hovers around 20% (haven't
looked at it in top yet but if there isn't a simple solution/reason for
this i will look into what sort of usage it is). The box is a home
router/server so at any 1 time isn't doing anything else that i can see
would account for this. I also noticed that this usage increase does not
occur with traffic coming/going on the inward facing interface, which
regularly has transfer speeds around 10 MB/s.

I'm running a AMD X2 3800+ with 2GB ram. Is this sort of usage normal?
Is there something i can do to bring it down?

Dylan