When your Mac is running on low battery, there are a few things that can happen that may make it seem slow. First, some of the system processes may be put into a lower-power mode in order to conserve energy. This can cause your Mac to take longer to complete tasks or to respond more slowly to input. Additionally, your display may dim or even turn off completely to save power, which can also make it seem like your Mac is running more slowly than usual.
In general, though, you shouldn't notice too much of a difference in speed when your Mac is running on low battery as compared to when it's plugged in and fully charged. So if you're finding that your Mac seems slower than normal when its battery is getting low, it's likely due to one of the other factors mentioned above rather than an actual decrease in performance.