As a point of reference, I did a build of the PB 12.6 version of the app. It is slower than previously also. Both PB versions are now building in 17-18 minutes. It appears it is something in my environment.
I spoke with my management and they will not allow me to submit source code for your testing. Beyond that, do you have any experience with what might affect performance of the build?
If not, I will continue to pursue what might be affecting performance here.
If PB 12.6 is slow it is either a regression bug introduced by SAP or it is something in your environment. We are not aware of widespread issue so if it is a bug it's got to be an uncommon usage/scenario. This is why I was suggesting you provide a test case. Anyway, sorry can't help you more than this.