That one feature may be fast but anyone who ever used Outlook with multiple shared mailboxes, mailboxes over a few years old, archiving to PST files, etc. will know that Outlook Classic could hardly have ever been considered fast.

Although I don't love it and avoided it for many years I've noticed a lot of these issues went away with the new Outlook. I can load and unload shared mailboxes on demand and I stopped worrying about cache mode settings about five minutes after I switched to it.

It's not great, it's not terrible, like most software I use. But I would have always said the same about Outlook Classic.

Bottom line for me is that MS has Classic on maintenance mode and it's only a matter of time before it's gone.