I rode a 38SK on a road 100k and it wasn't quite as bad as I thought it would be. I have a 200km gravel route, which is probably 2/3 paved, so the SK's get a decent amount of road riding. I wouldn't pick them for road tires, but they never felt like they were rolling through molasses, like a 25mm Specialized armadillo. Although now all the shops are carrying the + model that probably has changed for the worse
The problem I have with the GK slicks is the tread wears quickly. Rene Herse are said to be better in that regard, but they are quite a bit more expensive.