Yes, no problem. Even models with millions of polygons work smoothly. The very first time you start a measurement in a heavy scene, Pro Measure needs a few seconds to get ready, you will see two small blinking dots next to "Multi Measurements". Once they stop, you can measure as usual, and from then on everything is instant.

Yes. All measurements are stored inside your .blend file. Save as usual, and when you reopen the project everything is right where you left it, the display even turns itself back on automatically..

Sometimes old files are left behind and block the installation. This fixes it:

1. Close Blender.
2. Delete the ProMeasure folder. You find it here (replace with your Blender version, e.g. 4.5):
   Windows: %APPDATA%\Blender Foundation\Blender\\extensions\user_default\ProMeasure
   macOS: ~/Library/Application Support/Blender//extensions/user_default/ProMeasure
   Linux: ~/.config/blender//extensions/user_default/ProMeasure
3. Start Blender again.
4. Drag the zip file from Superhive into the Blender window.