EDIT Solved the problem below by switching to another driver in X2 and back again. Odd, but it works now!
When I connect my Oxygen 61 to my sons laptop there is no lag when I play something. When I connect the Oxygen 25 to this laptop, there is lag. And that's with the exact same latency settings! Anyone has a clue what this might be...? He is using Sonar X1 on Windows 8.