I've got the last edition Premier Big Boy, 20-3808-1, and I did notice it ran a bit jerky below 3-4 mph. I swapped out the stock no name Chinese motor for a ball bearing 9234 Pittman and it runs so much smoother. It'll crawl down to 1-2 mph without issue, and it's far from broken in yet.
Lionel's Canon motor isn't much better. I swapped the Canon in my Legacy 2031170 ATSF Northern for a 9433 Pittman and the low speed performance pulling a train was greatly improved.