Hello !
Deployed 9.1.2057.0 across numerous stations yesterday (around 400) and received some level of complaints in Outlook performance. Mainly displaying 1 email and switching to another took 5-10 sec.
As stated in July, the fix involving turning off "modified email" work in email client section of advanced config worked.
I will also empty the sync issue folder to see if any improvement has been made in the sense too.
Still observing... but they can work now.