When I scan trhough my imported raw-files in Lightroom, 50% zoom shows blurry/pixelated pictures. However if I first start with 100% zoom the picture looks sharp after several second and still do if I then go to 50%. Why can't I use 50% zoom directly?
From your description it would seem that you're not building Standard-sized previews when importing your files. Make sure that you have 'Standard' selected in Import window as shown in below screenshot. You'll also need to use 'Build Standard-sized Previews' for any that you've already imported. This option can be found in the Library module under Library > Previews