Two particular deal-breakers:
Most of our layouts contain many hundreds of stacked objects. When in layout mode we can no longer click in the center of transparent objects to get to objects underneath. We can only select the frontmost object, transparent or not. I can't imagine what this will do do our productivity.
We use conditional formatting extensively. In one case we highlight a portal row, passing the record id as a parameter from a button calling a navigation script. We conditionally format the highlight on that particular record. In v. 11, this takes about .33 second, even with loaded portals.
In v 12, we clocked this same script at just over two seconds, with only three records in the portal, making it essentially unusable.
We are very concerned.
(We develop on MacBook Pros v 10.6.8)
TIA for feedback.