When I changed computer and upgraded to Ac27, my Redshift render scenes suddenly became very slow. When monitoring the cpu and gpu usage I could see that Redshift was'nt using the gpu at all. This was strange since Redshift renders on the graphics card and not the cpu. My new graphics card was better so I had expected the opposite; a boost of render times. I searched this forum but could not find anyone adressing this issue. However, if someone out there experiences the same problem here is a link to a solution. It is in a Vectorworks forum but I followed the steps and now Redshift is faster then ever.
Tip: Fix for Redshift not using GPU - Rendering - Vectorworks Community Board
Bonus tip for even faster Redshift rendering is to close every other program that might take gpu resources, for example other instances of Archicad, Photoshop etc.