What does -1.13 times faster mean? I'm reading High Performance JavaScript, and I think the graphs in one chapter are just plain wrong. Here is one on Google Books.
The y axis is "Times faster", and it runs from -1.5 to +4.0. Now, I would have thought that "1 times faster" means "no faster", "2 times faster" means "twice as fast", and "0.5 times faster" means "half as fast"/"twice as slow". Have they just got completely confused in that graph, or is it me?

 A: Sorry for the confusion, but you got it absolutely right.
2 times faster means two times as fast
A takes 100 ms
B takes 200 ms
so A is two times faster than B
This particular graph is probably the most confusing taken out of the flow of the previous ones in the chapter. This one shows an optimization that most often you shouldn't bother. Especially when there are cases of stuff being 100 times faster. And especially when it's not consistent among browsers.
The point was not to worry about micro-optimizations (unless it's critical to you and you've done all else). So 20% faster (or 1.2 times faster) is probably not worth it most of the time when there are other optimization that will make something 10 times faster.
A: I would say $-1.13$ faster means 1.13 slower. For example $-1.13$ faster than 160 mph is $-180.8$ mph, i.e. 180.8 going in the opposite direction.
A: Suppose browser $A$ finishes loading a given page in five seconds, while browser $B$ completes in half the time.
How many times faster is $B$ than $A$? Well, twice, of course. But what about the inverse? How many "times faster" is $B$ than $A$?
One way to answer this is to assume that "$-x$ times faster than" just means "$1/x$ times the speed of". The intuitive problem with this is determining how much faster something is moving compared to something moving slower than it. It's just a really awkward way of putting it, but basically we can look at the number of "times faster" as a negative or a fractional value.
They're both more or less valid interpretations.
Now if we ask the question, how many times slower than $B$ is $A$, we get the original answer -- it's twice as slow. 
So how many times faster is it? It depends on which interpretation we're using. It's correct to say that it is -2 times "as fast" since it is twice as slow; but also (and more intuitively) correct to say the speed of $A$ is 0.5 times the speed of $B$.
A: There are many abuses like this in the world.  I have seen things on sale that were 400% OFF!!!  Would they give me 3x the original price if I took it?  Maybe -1.13 times faster means that you send them 113% as many bits as you received in the base case.
A: I suppose Skilldrick is right, and $-x$ times faster is $x$ times slower or $1/x$ times faster. For example, if A takes 2 seconds and B is $2$ times faster, B takes 1 second; if C is $-2$ times faster, it takes 4 seconds. And A is $0$ times faster than itself.
A: Strictly speaking, $-1.13$ times faster would mean that the program is doing things in reverse order. And Opera would completely stand still. Since that can't be what they really mean, it must be an abuse of language as has already been pointed out. 
Could it be though that the original scale was logarithmic and that the person then just interpreted the numbers incorrectly? Which would mean that the graph is correct, but the person who added the 'times faster' doesn't get the concept of logarithm?
So, $-1.13$ should be $a^{-1.13}$ in some suitable base, probably base $10$, $2$ or $e$?
