Running a managed replica for your mail is great.  Recently I found a cause for mail slowly opening: If the mail you are opening contains an attachment or image, and is older than the number of days specified in CacheDBInitialFullDoc, it will open slowly.

Setting CacheDBInitialFullDoc to a very high number solves this.

Note:  If CacheDBInitialFullDoc reverts to the default 30, you are bitten by this bug: https://www.ibm.com/support/pages/apar/LO77602

If you have a Desktop Policy doc with no managed replica set in the mail tab in 'local mail file', the Managaed replica settings are still enforced.  Open up your Desktop Policy, go to the mail tab, in 'Local mail file' change to managed replica, set CacheDBInitialFullDoc to 99999, and set the values you don't use to "Don't set'. 

Have fun