That's very impressive running times Dave.
I had a problem with slow running on my pre-owned Bertie. On my small Faller eTrain portable layout (15" radius curves) it would slow down in the curves & speed up on the straights, whereas my Millie ran steadily. After checking the wheels back to back, etc. it was suggested that it may be the valve timing. I did adjust one valve rod by a 1/2 turn which did improve the slow running but still not perfect. My Roundhouse Jennie (my first brand new Roundhouse loco, all others are pre-owned) ran perfectly out of the box with some impressive slow running. I took it apart to see how the valve timing was set up, and subsequently I've adjusted some of the valve timings on my other Basic Series locomotives. The Bertie now runs much better at slow running, but it can suffer from the gas tank getting heated up during a run requiring small adjustments to the gas regulator valve.
Chris Cairns