If you use this command
/give @p potion{CustomPotionEffects:[

]}
You get a potion that gives you speed 1 instead of -4.
This might be intentional, but I'm pretty sure a while back negative amplifiers worked and a speed -4 potion would have slowed you down.
I also noticed that an Amplifier greater or equal to 128 has the same effect, but at Amplifier 256 and greater it sort of warps back to 0 and Amplifier 257 would act the same as Amplifier 1. Similar effect for negative amplifiers at -129 or below.