I have a client who said his https://www.usc.edu/office365's Outlook is slow in responding (a few seconds) and loading his e-mails, viewing attached files, clicking on print, etc. He has a working fast cable Internet connection and using his month old 2020 13" Intel i7 MacBook Pro with Big Sur v11.1 (clean installs, but with migrated datas from his former 13" 2012 MacBook Pro's Mojave v10.14.6). I already tried clearing his histories and caches, but that made no differences.
I narrowed down the slow Office 365's online Outlook's emails in Firefox v85. It involves with their attached Office files. Firefox's about:performance and about:memory show crazy high usages with connected online editors for Word, Excel, etc. Safari has the same issue, but not so bad as Firefox. Chrome had no problems at all. I even tried another macOS account, upgrading macOS Big Sur to v11.2, and refreshing Firefox. :(