We've created a layout that is a template that we'd like to use for a dozen maps.
Here's what we are doing:
We've tried on multiple computers without success. If it makes any difference, the project and data are all accessed via UNC paths. Looking at the performance tab of task manager, nothing (GPU, Memory, etc) seems to be maxing out.
What version of ArcGIS Pro are you using? A lot of work was done in this area over the 2.3 and 2.4 releases. It is possible that something was missed. Can you contact Esri Support? They are better equipped to handle troubleshooting steps and gather bug data. They can be contacted at https://support.esri.com/en/contact-tech-support.