Safari? Lightest-weight usably-well-supported browser around, by a long shot. Preview? Outstanding for a bunch of reasons, including that using it is the only time I've been happy to receive PDF files. Pages, Numbers, Keynote? More than enough for everything I do, stable, and I like that I can leave them open in the background for weeks and they're light enough that I forget they're there. Notes? Not having a built-in export function is annoying and I wish I could use markdown formatting, but it's so good at everything else that those haven't been enough for me to switch to something else. Hell, I even like the calculator better than most others.
If you have Excel running, open the Activity Monitor and find the app in the list. Then look at "Kind" column, you will see "Intel" listed. So that's why Office365 are sluggish on it.
Thanks for letting me know!