Nilhan, I believe I found my problem which affected two machines running ML 10.8. I found two Outlook preferences files - identical but for date - that I believe were causing a conflict.
It was intuition that led me to search for this. I noticed the splash screen text was mentioning that Outlook was looking for identities on startup. I'd never before noticed this, or it happened so quickly that I've never seen it. In any event, I did a search for all files name "outlook" using the program EasyFind. When the search completed, I sorted the found files by type. I was looking for duplicate preferences - and I found them.
There were two folders named "Office 2011." One was in (user) > Library > Preferences > Microsoft. The other was in (user) > Library > Application Support > Microsoft > Office > Preferences. These two identically named folders each had similar items enclosed, including a file named "Outlook Preferences." The dates on the "Outlook Preferences" files were not the same, although they were the same size. The date on the older file corresponds to a Microsoft Office update which was supposed to move things to locations required by Mountain Lion. The older files (some of the file dates were current - yet duplicates in name of others) were in the location: (user) > Library > Preferences > Microsoft.
I moved the "Office 2011" folder located in (user) > Library > Preferences > Microsoft to the trash. I did not empty the trash just out of caution, although I probably will ...or even archive them via a zip file, etc. Next, I restarted my machine. After logging in, I rebuilt the Outlook database using Microsoft Database Utility (hold the Option key and start Outlook to start it). I then backed up my Main Identity to a separate disk as another caution. Next, I once again restarted.
After giving the machine a little time to settle down, I opened Outlook 2011. Outlook now opened many times faster then before. I would venture to say that it is now opening as quickly as before the 10.8 ML install. All functions within Outlook also seem to be at normal speed.
This process was repeated on another Mac with a new install of 10.8 ML and Office 2011. I had tested Outlook on that machine, and it was slow. After the deletion of the duplicate preference files, Outlook was fast as ever - noticeably very fast. It was a vast difference on that machine. The Main Identity database is much smaller on that machine than the previous machine, so it should be quicker. Still, it was a tremendous difference in speed after I dumped the older preferences on it.
I am no expert on the workings of 10.8 ML compared to earlier versions of OS X, but I do understand that certain preference files are now required to be in a certain place ...different than earlier versions, or perhaps just mandating that all preference files be in a universal spot. Something like that... In any event, 10.8 ML was not the problem, but seems to have brought a problem to light. If I am going to point a finger, I will point it at a Microsoft Office update that didn't quite do its job correctly. There was trouble with an Office update not long ago, and I suspect I was a victim of that.
Maybe you are, too. Hope this helps.
