Soul speed inverts the effect of Soul Sand, but there's more to it. The pic above is a cheap chart, and it reveals something interesting.

The numbers above are from test I did. They are how fast, in hopper items, I can move on foot. The lower the number, the faster I ran. This was done on a 100 block track.

Here's how to read the chart.
Ok, so it looks like PMC wants to display my chart at 1/4 the size for whatever BS reason, so here are the important numbers:
No Bonuses: 43
Speed II: 30
Soul Speed I: 30
Soul Speed III: 25
Speed II + Soul Speed III: 18

As you can see, Speed stacks with Soul Speed! You can combine the two to achieve even faster speeds! Of course, there are still faster ways to travel, but I thought it was interesting for two reasons.
1) Other speed bonuses don't stack.
2) This new "top speed" is almost 2x the speed of Speed II alone.

Furthermore, Soul Speed I is the same as Speed II, so even those cheap boots from bartering are useful.

If you want to see a properly displayed image, then click the link here.
The keys are:
Grass: No Bonus
Sugar: Speed I
Glowstone: Speed II
Soul Sand: Soul Sand
Soul Soil: Soul Speeds 1 - 3