Further investigation, checked Compiler settings to other projects I have in Objective-C they look similar and the differences do not look as if they would matter. These other Obj-C projects compile as fast on the Ultra and the time taken in the same ballpark. The only thing I see which is a big difference with these slow Obj-C files is the totally crazy size of -I includes in the compile statement. As far as I understand it, a lot of these -I includes are auto-generated from the various frameworks we need to include in our build via Carthage. Looking at a single file I can count 2825 -I statements!!!! But why would this affect an Ultra system more when compared to an M1 Pro machine? Perhaps I'm going down the wrong rabbit hole, but that is the only thing I am seeing different at the moment.

Yesterday I wiped the Studio clean and reinstalled a fresh build of Monterey. Reinstalled the same software and copied the latest account from the MacBook Pro. I'm still seeing the slow objective c compiles!