There's no standard, or even agreement. Generally, people mean "compared to other cameras of that general class". Or maybe "compared to a sense of all cameras available now" — it depends on the context.

The answer may even change over time — a camera may be fast compared to the competition when it comes out, but a few years later be slow compared to the new generation. And, by itself, this is missing another crucial element: slow at what? Bad at tracking moving subjects? Slow to acquire focus lock? Hunts around in low light?

There's also no sense of scale without any context — does "slow" mean "excruciating to use" or does it mean "undetectable without measuring, but measurements show...."? Reviews often get caught up in the latter, because the Internet selects for reviews which seem serious because they measure things and provide numbers (as opposed to objective, in the field use), and the secret is that all cameras made today are extremely powerful and awesome, so there's incentive to make a big deal of small things. This is compounded by the disease imusthavethebeststuffitis, which leads to a the hobby of comparing and contrasting such things on forums — which, sure, can be a fine hobby, but ultimately less interesting than actually going out and making photographs.

Basically: you need context; "slow" doesn't mean much by itself.