I'm always baffled when people complain that Outlook is slow. The corporate exchange server I am connected to is not even in the same country as I am. And outlook starts in about 7 seconds in a cool boot (ho often do you that, once a week?). That's still too long, but it's not 2 minutes. My mail is all on the server, no local
.ost file. (probably the same for you too, since you switched to an (no longer developed afaik) imap client)
But there are some crazy corporate spyware/management stuff that corporation install on our PCs that slow things down, and for sure "going rogue" and using another mail client that is not IT-managed works around that.