Building RAF thumbnails on import very slow

Hi there, I am struggling with a problem that I tried to ignore for a while. But after haveing tested a LR competitor software (and having seen how fast imports could be) I am frustrated about this issue. So I hope for a solution on that.

In the import dialog, for my Fuji RAF files, it takes a long time to generate the thunbnails to chosse the files to be imported. Each thumbnail takes verious seconds to generate. Let´s say, for 30 Files it may take about 2 -3 Minutes. Sometimes some of the thumbnails seem to not generate at all. But: whem I click on the single view of one file - the preview is immediately there (obviously the embedded JPG) - going back to the grid view, it still takes forever and most of the thumbnails are still gray. This makes importing almost impossible. 

I already reinstalled LR from scratch. I tried with a new catalog. Issue always the same.

Any ideas or recommendations on that? 

MacbookPro Late 2016, BigSur