The forklift has seen little use since we got it, maybe 10 hours of light use total in the 3 weeks, and I haven't noticed the forklift being sluggish at all. I don't believe that the batteries are too low.

And no, I'm plugging into the battery pigtail side. One of my employees plugged it into the wrong side once so it's not unthinkable that someone might do that (on our other forklift). He plugged in the forklift 50+ times and one time he just blanked and plugged it into the wrong side.