Target velocity not reached

I was doing some test flights a few days ago and encountered a problem whereby the loiter speed I had set was not being reached. At 5m/s it worked fine, but at 10m/s the copter was only achieving about 8.5-9m/s and taking a while to get there. Looking at the logs I can see in the PSC fields that target velocity and actual velocity match very nicely, but the target velocity is less than what I’m expecting.

Two logs here: 42.bin has some 5m/s and 10m/s runs, 43.bin starts with some 12m/s runs.

Why is the target speed not being reached? What am I overlooking?