Be sure you imported with the "Expiremental" .ply import option in Blender 3.6. The previous version of .ply import ignores color data.

If you have autosave enabled Blender will try to save every so often. This can cause a delay between 2-8 seconds depending on how large & how many point clouds you have brought into your scene. This can be a large amount of data to save onto your hard drive.