Generating thumbnails

Why on opening Substance Painter do the shelf thumbnails all need to regenerated, indicated by the generating thumbnails message? Admittedly I have a lot of assets in my shelf, some of which are complex, but generating the complete set every time I open the application just adds unnecessary delays.
As a comparison, when I open Lightroom I have the equivalent to 20,000 thumbnails (previews) instantly available they are not regenerated every time. 

I can fully understand the need to generate fresh thumbnails on new or modified assets, or even on all assets if the application has been updated or the 3D engine changed. However, surely the principle of a thumbnail cache should be that unchanged assets can use a previously generated thumbnail.

So the idea is : To use previously generated thumbnails unless the asset is new, marked as modified, or the application has changed.

Dave