I have the polar opposite results. I had the Presonus 44vsl, and had nothing but problems with latency. I was in contact with Presonus tech support for 2 days, and finally returned it, and got a Focusrite 18i20, and never had any issues.
Yeah, the 18i20 is very nice. Head and shoulders above the 6i6. Don't get me wrong, I wanted the 6i6 to work, but guess it just wasn't in the cards.