Does a low battery on a computor slow it down?

Only if the computer has a power saving function that kicks in when the battery is low. Most do.

Depends if you have power saving set to turn on when it gets low. You can normally turn this off in the settings and you will be fine.

No, as the battery on the computer gets low, its fine.

On a lot of notebooks it will trigger power saving features which may include throttling down the CPU, so yes it can slow it down.