BrianBeck

It's actually because Auto Roof Returns are turned on for the walls in the Fireplace, as well as a bunch of other walls in the plan where it probably doesn't need to be on.  The fireplace walls are the ones causing the slowness though.  If you select them and turn off Auto Roof Returns the plan is no longer slow to build 3D.

Thanks Chief ID.
 
I took a look at the your plan and was able to reproduce the issue.  The problem is caused by the Sun Follows Camera setting.  So for now if you turn that setting off your views should work at the same speed as X14.  We will look into the cause of the issue in X15 and hopefully get it fixed in a future update.
 
For others that read this, the problem only effects PBR views on systems that do not support ray tracing.

It is being used in your Dormer defaults.  If you change the wall type there to something else you will be able to delete it.
 

Looks like there is a new feature that came as part of the upgrade that removes followers who have had no activity on the forums for over year.
 
We could change the timeout or set it to never, but I think the current default of 1 year seems reasonable.