FYI,
It has now been confirmed by Grisoft, that support for v7.5 will continue until the end of 2008,
NOT as stated on the pop-up messages that you may get.
I have uninstalled v8 and re-installed v7.5, and speeds have recovered back to normal.
If you are experiencing system/internet slow-downs since upgrading to v8, then it's your choice, but I recommend re-installing v7.5. It will, at least, give Grisoft 7 months to sort out the issues.
Keep an eye out on their forum at :-
http://freeforum.avg.com/