A Commodore 64 can keep up with even the best human typists. How is it even remotely acceptable that any application running on a GHz+ multicore CPU can't?
Yes but it's disappointing that it's slower than the 2013 version on the same hardware.
The issue seems to be down to your network connection. As Outlook is sending or receiving emails and making other service calls you experience a slowdown. Onsite our connection is very slow and everyone experiences this problem. Additional bandwidth was not considered for the O365 and if you monitor your service calls with fiddler for example you'll see constant traffic to the O365 servers.
We're talking about pressing keys on a keyboard and the corresponding letters appearing on the screen, maybe even with spellchecking. This has been computationally achievable for decades. It's not something where you need a state-of-the-art PC in order to be able to experience it.