Importing was quite quick - around 4 hours. The files were in RAW format and stored on a NAS system. I consider that to be a very good performance. Meanwhile I was able to work with darktable.
I think Darktable has a lot of options to work with RAW pictures. Not bad.
But the performance when working with these files is very bad. Lightroom ist very much faster. So for me it was a try but I will give up for the moment.
Thanks again for your help!