Dunno. The thread I linked to alluded to the "old drift problem," which at times seemed linked to the low-speed issue but was probably unrelated. I've never experienced the problem first-hand, likely because I mainly use my Vista on the bike at speeds higher than a snail's pace.
Seemed as though Garmin was trying hard to maximize the HCx's high-sensitivity, low-discretion reception through dense foliage without giving the appearance of less-than-pinpoint accuracy, resulting in these drift and speed problems. In trying to filter out spikes in location through software, I believe they gave up some off this small scale accuracy. Sort of like trying average a bunch of shotgun pellets to determine where exactly the gun was aimed.