The problem with Outlook isn't that it's slow. It's visually cluttered and difficult to see (why, oh, why, is everything gray instead of black?), the functions are hard to find and figure out, and the functionality is clunky. It is also incredibly annoying that you can't scroll down the calendar, but must use your mouse to select the next month. Frequently, I'm trying to look at a sequence of events that straddles two months.
My current peeve, which is what brought me here, is that I have a recurrent appointment every Tuesday. Outlook highlights every day of the week, not just Tuesdays. Why would you do that?
Outlook has been around for a long time, and I used to use it without complaint. Windows 10 calendar and email were so much better, and if you had to pick only one, I wish you would have picked the good one. But Microsoft not only made us all move to Outlook; they've changed Outlook to be clunky and user-unfriendly.
I'm hanging on to my Windows 10 computer for as long as I can, since Windows 11 looks like another effort to make things better that just makes them worse. I wish there was a third option, because I don't want to switch to Apple but I sure am getting tired of Microsoft's failure to respect how much work is invested in knowing a system, and how much wasted effort is created when MS decides to change things up just to be new..