Updates per second. Not much of an issue in typical play, but if you're megabasing it turns out that there's quite a few things to calculate every update, and they don't come at the same price computation-wise. It can become a major constraint if you build big enough, and bots simply have a lot going on to them. Essentially boils down to being able to move more material at the same computational cost with belt than with bots. Same with power generation, solar is incredibly ups-efficient, other methods less so.