When you add speed 128 using /effect, then you get slower. However if you also add slowness 127 using /effect, then you get a lot faster than with speed 127. It also affects chunk loading / terrain generation - I bumped into a village when using this. The village looked completely destroyed, and when I created a new world with exact same settings, the village were normal. I have attached a screenshot of the village.
If you're running fast enough, some chunks will even refuse to load, and you need to re-load the world (F3+A doesn't work) to get them to load anyway.