Optimize the new version more.
Using the newest version of Google Chrome and a pretty beefy gaming laptop, the new Outlook just feels a lot slower. I tested my theory, and I am correct. It took me an extra minute to open Outlook, open an email, and download a document from that email.
Old outlook: 3.25 minutes
New outlook: 4.27 minutes
There isn't much of a difference, but this does show how much of an effect it can have on slower hardware. That minute could be maybe even an hour with an older computer.