There is an air-rate limit added to help smooth how fast boost comes on for smoothness, and that's what appears to be limiting the F-Series pickup trucks. Your air-rate is fastest in 1st gear typically as it's your quickest pull.
The F150 has this calibrated in, but all of the Fusion definitions I've seen have this calibrated out. If you could get us a log and tune of it happening on your Fusion, we can take a look and see if there's yet another thing in the way!