The freezing action begins right after the app opens and sometimes when you first take an action. I don’t get a spinning wheel but everything freezes and the seconds on the Mac clock stop. I can, however, move the mouse a bit. After a few minutes then any clicks I did rapidly process and it freezes again within seconds. This pattern then continues to repeat.
I just installed the upgrade on another MacPro this time a Late 2013 model and so far have not experienced the same issue. It does appear I am getting significant performance improvements for H.264 encoding which is good to see although the file size has grown considerably compared to the same file done with 4.1.3 using the same settings (something possibly for another thread).
I'm working the issue with Apple and they are exploring a possible compatibility issue with the graphic cards installed. The cards on the box having the issue are an ATI Radeon HD 5870 and NVIDIA GeForce GT 120 driving 5 ACDs ranging from 20" to 30".