Oh dear, this project is so dead! I do really not understand, why you're not give away projectlibre to the open source but keep it in a coma state, like you do!
Sadly the project owners do keep it closed instead of opening it up to the community :-(
Same issue here with 20.04 and 1.5.7beta and file dialog is quite slow to open
Same issue here with 20.04 and 1.5.7beta
And, where's the "ProjectLibre license to be found? The code states: Alternatively, the contents of this file may be used under the terms of the ProjectLibre End-User License Agreement (the ProjectLibre License) in which case the provisions of the ProjectLibre License are applicable instead of those above. If you wish to allow use of your version of this file only under the terms of the ProjectLibre License and not to allow others to use your version of this file under the CPAL, indicate your decision...
There's a long standing PR on github fixing this issue: https://github.com/claur/ProjectLibre/pull/2
There's no support as you can see from long standing bugs, discussions and feature requests https://github.com/Uatschitchun/LibreProject/discussions/2