I read that the right way to import my Mac Aperture library was with the import plug-in that is part of the desktop Lightroom. So I did. It took days.
But now I realize that I had TWO desktop versions -- the old Lightroom 2015.14 (which is what I used) and the newer Lightroom Classic 8.0 (which I think I should have used).
Q1: Should I toss out the library and redo this from scratch using 8.0? Does import using 8.0 do anything that the 2015.14 import does not? (Happily, I have not done anything with the library so redoing the import is merely inconvenient.)
Q2: If the import I did is fine, I presume next step is to make 8.0 use the library -- how do I do that?
Q3: If I do have to repeat the import, would like some tips -- the import I did previously ran slower and slower and slower and I want to do what I can to help it perform. I have lots of disk space, if that matters.
No, don't re-do anything. Open the Lightroom 2015.14 catalog using LR 8. In LR 8, use the command File->Open Catalog and select the LR 2015.14 catalog file.
DJ has you pointed in the right direction.
Since you have imported the photos into the 2015 catalog, I would open that catalog with the 2015 version of Lightroom to verify that all is good.
Once you are satisfied with it, Close that version of LR, open the v8.0 Lightroom, open the 2015 catalog and let v8.0 migrate that old catalog to the new format.