In general, it's tricky to say "N percent faster" and assume that people are all saying the same thing.  There are three distinctly different things that could be meant by that.

If you are just measuring the time it takes, then you're measuring the time it takes to do *something*.  Whatever that "something" is, it's what we're trying to speed up.  So, doing it in *less* time, is valuable.  What's interesting is: "How many *something*s can we do per second?"

The percentage numbers are a percent diff, not a percent change.  So, its `(a - b) / ((a + b) / 2) * 100`.  Let's consider the example where we have node-a that finishes in 300ms, and node-b that finishes the same task in 600ms.  For simplicity of math, let's say that the task is simply 1 job, so we're doing `bench.start(); doThing(); bench.end(1);`

```
A: 1 / 0.3s = 3.33333 things / second
B: 1 / 0.6s = 1.66667 things / second
%: (b - a) / ((a + b) / 2) * 100 = -66.667%
```

In other words, A is different from B by a factor of 67%.

If we want to measure percentage change, it'd be `(a-b)/a * 100`, assuming that A is the "current" and B is the "new".  Then we'd have:

```
A: 1 / 0.3s = 3.33333 things / second
B: 1 / 0.6s = 1.66667 things / second
%: (b - a) / a * 100 = -50%
```

and we could say that B is a 50% speed regression from A.

The third thing it could mean is the percentage change in the other direction.  In that case, we'd just switch the denominator from option 2:

```
A: 1 / 0.3s = 3.33333 things / second
B: 1 / 0.6s = 1.66667 things / second
%: (b - a) / b * 100 = -100%
```

and we could then say that A is 100% faster than B.

Since the percentage change is more commonly used in conversation than percentage difference, I think it makes sense to update the compare.js to do that instead.

If the values are not consistently higher-is-better, and consistently a measurement of operations/time, then none of this works.  The compare script has to know that sometimes 600 is a worse score than 300, which makes everything less consistent and harder to reason about.