What does -1.13 times faster mean?

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.