This is not entirely true. it affects all apps (not just MS) as they all open slower and the screen icon draws when I open Catlina are slower.
I have an older MacBookPro. I upgraded to Catalina and after install I was so impressed with Catalina, I made it my Primary partition to boot to. Not only did Avast slow down my system, it made it almost unusable from start up, every single app, the entire system, and added 1 minute to ready, longer than before I installed Avast. Not only that, after I uninstalled Avast, it left a residual slowness which is aggravating, like the apps bouncing when starting 6 times or more, every single App and Entire system bogged in the mud with the wheels spinning, looking for traction. Absolutely hopeless and I am being kind, because now, I ave to reinstall Catalina to be rid of the residual slowness, which will take all day.
Before my bad experience, Catalina was "blindingly fast", blink and you would miss any task, right now, before re install, it is like a slug and I hate the entire MacBook Pro, and want to throw it through the window.