This is a simple mod that adds a keybind that toggles a run speed modifier. You can change the modifier in the settings - it can take any value from -1 (-100% movement speed, ie you can't move at all) to a very high number.

I mostly use this with a negative value in my pY run where I have very fast movement speed tiles and I actually want to slow down my movement sometimes. But you can also use it to get around your base quickly if you like.