Hi! It's hard to tell without knowing the specifics.
What exactly are you comparing? Is the scene empty? Etc...
Hey Houska, thanks for the reply.
It is a very simple scene. White walls, a kitchen, parquet. That's all.
When the IR starts, it needs way more time to build up the picture when Corona Sky & Sun are loaded - compared with HDR.
If I may, I would upload you the scene files.
Where to do that?