When I was working on an image in Lightroom I recognized that the image is compressed (see image below) but I always record my images lossless compressed. Also the image seems to be fine when exported so it is just the preview in Lightroom. Is this something that can be deactivated?
While you not provide details about the images you work with, I can only answer in a very general way.
I assume that you are working with raw images. As raw images are not an image per se, it has to be interpreted by Lightroom. This images is either generated on the fly or pre-generated as a preview.
You can set the quality of the preview in Lightroom. You find the default setting in the Catalogue Settings / File Handling / Standard Preview Size.
You can also generate higher quality previews in the Library Module: Library / Previews / Build 1:1 Previews or Build Smart Previews
The final full resolution image will only be generated when you export an image.