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Disclaimer: I am neither Avast Team or even Moderator…just a poster like you…but some thoughts.
A lot of people claim there are other A/Vs that don’t do this…I have not seen them.
I also have fast PCs but the only time they are fast on boot is as you say…when you first buy them. 
Question, why does your friend have PC shutdown ?..why not leave booted ?..if you don’t like power drain have it go in STBY.
I ask because for me and “PCs” it is almost standard procedure/expectation from me that a PC takes a while to boot and “settle”.
I usually walk away from PC after login and let it settle…not just A/V but other tasks are banging away.
My only point is that that Avast is trying to keep you updated and protected and it has no idea how long PC has been shutdown…ie, how many updates needed. Essexboy showed a way to “delay” this above…did you try ?
I’m like you…don’t like anything slowing down my PC but A/Vs and background imaging software are the #1 software that do this.
I think the nature of how invasive A/Vs have becomes in order to protect your PC means they take more time to update as well.
Combine this with other competing tasks on boot and there is a slowdown.
So, with all this said I think it comes down to a level of "acceptability’ versus protection.
I personally don’t judge a PC’s speed on boot unless it locks or takes more than 5 minutes from login screen to home screen “settling”.
It is my own metric but as long as this “for me” is in this range…and more importantly the PC runs fast during settled operation…I’m OK.
Just my thoughts…not sure if it answers anything or not. 