Toshiba Intel P7450 2.13 GHz, 4 Gig Ram / Win7 (x64), AIS 6.0, Malwarebytes Pro, RollbackRx, Acronis TI, SugarSync (free).
Well your system is no slouch either and comparing with previous avast5 is a good marker.
Were you also using AIS with version5 ?
However, avast 6 does have more functionality, so I don't know if that might mean it is doing more scanning at boot if some of these applications are running on boot:
Malwarebytes Pro, RollbackRx, Acronis TI.
So their access to the system may well cause avast to do more scanning in avast6 than in avast5. I don't know how early the behaviour shield might be running as these two applications RollbackRx, Acronis TI may have it working overtime.
There were some instances certainly in the 6.0 beta tests with some drive imaging software, so I don't know if there might be something here in these two. Something that you could check with notepad, c:\ProgramData\AVAST Software\Avast\report\BehaviorShield.txt and see if there are any entries for either of these applications.
If so you could copy and paste the full path and file name into the BehS Trusted processes and see if it makes any difference.
I really haven't seen any noticeable difference in boot times from avast free 5.1 to 6.0.1000 on the system in my signature and my win7 starter netbook.