There is a known issue with Pro where there are many maps and/or layouts and/or SDE connections and/or ArcGIS Server Connections and/or Portal connections (any combination) that causes a project to take a very long time to load. Are you in that situation? Need very exact and specific details of the project you are trying to load, what are the options set to, is your default GDB and TBX pointing to a network share? What layers are loading slowly, where are they hosted, what version is the software hosting them? If enterprise data sources, are you using windows authentication? SSL?