I use Avast precisely because (for me) it causes fewer problems when it updates than other free security programs do. However it can still slow everything to a crawl at times. (This computer is normally left on 24 hours per day, so that I don't have to worry about it updating everything when I turn it on; it's usually done that overnight anyway).
As soon as you turn your computer on, open the Task Manager (via right-clicking on the taskbar at the foot of your screen). Click on the 'Processes' tab and then on the button marked 'Show processes from all users'. (If you're asked for permission to proceed, grant it).
Look down the 'CPU' and 'Memory' columns to see if there are any particularly high numbers in either. I'd suspect that you'd see 'svchost. exe' has a very large figure alongside it in the 'Memory' column. (That shows that your computer is downloading something big). If so, take a careful look at the other figures in the 'Memory' column, to find one that keeps changing every second or two. (It doesn't have to be very big but those constant changes tell you that it's what's making the call on svchost.exe). I'd strongly expect that it will turn out to be AvastSve.exe, which is Avast's updater working.
If so, you could
(a) just live with it and accept it as an annoying part of needing security software ;
(b) try uninstalling Avast and reinstalling it ; or
(c) try another anti-virus program.
If you find that other programs have very high 'Memory' (or 'CPU') figures, let us know what they are as they rather than Avast, might be causing your problem.