Lightroom CC 8.4.1 on iMac 2017 very slow

I needed to leave Aperture (Mac) to another application and decided for Lightroom cc. While most of the differences are quite easy to cope with, the speed is annoyingly slow.

The catalogue contains almost 40000 pictures or better it contains references to the originals, which are stored in a folder structure on the internal hard drive - the same way I handled it with Aperture. However, while I could quickly scroll up and down through the thumbnails in Aperture (even in the "show all fotos mode") in Lightroom this scrolling action is extremely slow - even worse, when I start lightroom it takes several minutes before any picture is shown on the screen - it is awkward.

I hope it is just a stupid setting I need to change and maybe someone has an idea which setting it might be. The Adobe proposals for performance optimization on  https://helpx.adobe.com/lightroom-classic/kb/optimize-performance-lightroom.html I already tried without success.