NoelCan,
I'm using Poser 8 on a 2008 iMac.  While I do have problems that are being fixed in SR1, I haven't seen the Internet slowdown that you've observed.  However, I'm running Mac OS X 10.5.7.

Just to make sure there wasn't anything slowing down my Internet connection, I ran Activity Monitor and selected the network display.  I also brought up the speed test page on Speakeasy.net.  Next, I launched P8 and watched Activity Monitor for network utilization (didn't see anything).  Then I ran the speed test on Speakeasy and got my full DSL bandwidth.  I loaded some content into P8, started a render and repeated the speed test.  Same full (normal) bandwidth results and no network activity except for the speed test.

I then opened P8's preferences and selected render in a separate process and repeated the tests again.  I just can't repeat your experience on my Mac.  Having said that, I haven't upgraded to Mac OS X 10.6.1 as you have.  My results may vary if I were to upgrade to 10.6.1.  Would you be willing to try the same sort of tests?  Perhaps they would reveal a clue as to what's going on.
Thanks and take care! :)
Lee