Whenever you see an image in Lightroom Classic, you are actually looking at a preview. When in Library module, you are seeing Library previews ( and if applicable smart previews). Both for grid view and Lupe view. You are also seeing previews in the Navigation Panel, and the filmstrip. When in Develop module, you are seeing Camera RAW CACHE files (in theLupe) (hmm, you will ask, but what about JPEG, think the same location, despite not needing conversion)
This is because Lightroom Classic is a Parametric Program, it is that way as to be non destructive to the photos.
Now you can delete the Library previews to save hard drive space. When you restart Lightroom Classic,those previews will be recreated as needed.
The previews forDevelop module should automatically get purged upon closure of lightroom Classic.
Just another Adobe customer; My Sys; APP: LRC 9.4, PS 21.2.1; CMP: WIN WS 16GB OS 10 v1909 (18363), mid 2015 MBPr 15” 16GB MACOS 10.15.6; 4K EXT DSPY; CAM: Canon 5D Mk III, Fuji X-T3
As I understand it- If you import your photos at a small preview size (especially 'Minimal') Lr has to 'build' a suitable sized preview when you want to look at the file full-size, or at 1:1. Building large Previews is the "loading".
You may benefit from creating 1:1 previews as you import. It should speed up or eliminate the 'loading' (Preferences can be set to discard the 1:1 previews after a set time to keep the Previews folder/file down to a manageable size.)
The standard-sized previews are a pyramid of several sizes, up to the size in 'Loupe view'. If you build standard-sized previews on import, then you should not see the 'loading' message as long as you are in the Library module and looking at images up to loupe view size (unless something has changed, which requires a rebuilt of the preview). If you zoom in to 100%, then a 1:1 preview has to be built.
The Develop module uses its own, different previews. No matter what you build on import, you will see a short delay when you go from image to image in the Develop module for the first time. If you go back and forth between images then that preview gets loaded from the Camera Raw cache, so increasing the cache size may help. How long that initial delay is depends on your hardware.