Weight is a big factor. I drove a '15 K900 a year ago. Compared to my Cadenza, it felt faster but not substantially so. Cadenza = 3700 lbs, K900 = 4500 lbs.

I recently took a '15 Hyundai Genesis 5.0 for a drive. Again, it felt faster than my Cadenza, but less than I expected. I then looked up the weight. The '15 Genesis 5.0 is at 4500 lbs just like the K900. I expected it to be less. It's Cadenza-sized (thought with RWD). I see the V6 Genesis is closer, at 4100 lbs.