SCVs harvest the slowest of all workers because they have the lowest acceleration. However, sometimes they don't need to accelerate and seem to jump to max speed. This happens on certain minerals, depending on how they are placed on the map in relation to the command center.
I observed this effect when I was trying to measure how many SCVs can mine a mineral for maximum efficiency. I defined idle time for each mineral as any frame an SCV is travelling and not harvesting from it. Dividing by the total time gives the idle ratio. As seen below, the ratios for 1 SCV per patch is about 0.5, so in theory 2 SCVs can mine from each patch at full efficiency.
Take note of the top 2 mineral patches. They both have had only one SCV (the 10th extra SCV stays around the bottom) for a while, and have stabilized to different idle values. The top one spends 48% idle, while the one under it is 55% idle. The SCV that mines the top patch is consistently moving faster than the SCV directly below it.
If we assume the difference is from the top SCV not having to accelerate, and that it is possible to micro the SCVs to move faster in mining, then 2 SCVs per patch should achieve perfect efficiency.
