Just working through some PUM patching of an HCM environment and the change package that was created was a little on the large size. In the past large change assistant templates cause extreme delays in trying to get change assistant to respond.
One thing you can do is increase the Heap Size of Change assistant. The easiest way to do this is to create a shortcut to changeassistant.exe and append the parameter: -Xmx1024m
In PeopleTools 8.54, I have also found that to minimize the size of the application, you can do an Initial Pass with just the source and/or target steps. This cuts out a number of steps and makes an easier package to work with. Also, you can disable the file deployment and do it the old fashion manual way and this too will reduce the template work load.
For example I am doing Image 9 to 12 HCM patching right now, and the file deployment is approximately 1/3rd of the entire change package XML file.