Same here ! Macbook pro 13 inch (early 2015) OS becomes slow, A lot of lag while multitasking. I have to keep monitor my application and close unwanted, I didn't do that in previous versions of OS. bigsur was a big mistake for me.

The below steps worked for me even my OS is Monterey. It seems there is a bug that outlook profile corrupt after updating Monterey to 12.1 If you are using macOS Catalina, try to fix the issue by deleting the Outlook profile and adding it again as follows: From Finder, open the Applications folder. Ctrl+click or right-click Microsoft Outlook, and then select Show Package Contents. Open Contents > SharedSupport, and then launch Outlook Profile Manager. Select the profile that you want to remove, and then choose the Delete the selected profile button. Restart Outlook and add the profile again. https://docs.microsoft.com/en-us/outlook/troubleshoot/outlook-for-mac/useful-tools