It probably has to do with the way that you have your inbox configured. If you have all of your folders under the inbox, it takes longer to sync during startup, and this also delays the ability to sync newly received emails. It also has to do with the way that Outlook 2011 is designed, based off of the version 1 releases, the next version should rectify this.
I too have moved away from Outlook, but mainly because I get all sorts of sync issues, caused by my CRM account being tied into my Outlook (needed for work). I felt that it was a resource hog as well (but could never prove it through Activity monitor). Turns out it was because of my folders being under the inbox and just the nature of the current design. I just felt that it was bloated and slow due to Mail and my iPhone receiving quicker. Like I said it was due to my configuration and the nature of Outlook, not the use of resources.