We’ve seen instances where malformed catalogs can cause excessive memory usage at idle. (Especially on older, larger catalogs. This is normally confirmed by creating a new minimal catalog and seeing if memory climbs. If it doesn't you may have an aberration in your catalog. If you want to test this hypothesis please try the following.
- Select All Photos in Catalog Panel of Library Module
- [Ctrl/Cmd]+[A] to select all
- Right-click on any photo and select Stacking>Expand All Stacks
- [Ctrl/Cmd]+[A] to select all again
- File>Export as Catalog...
- Save As: TestHighMemoryUsageCopy
- Where: Desktop
- Do not check Export Negative Files or Build/Include Smart Previews
- Check: Include available previews
Open the TestHighMemoryUsageCopy.LRCAT file in Lightroom and attempt the same operation(s) that caused the high memory usage.
Does the high-memory usage repeat with the test catalog?