I know that Phase Detection autofocus is faster than Contrast Measurement autofocus.
Is there a spec that can tell me whether one DSLR or compact camera can achieve autofocus on a subject faster than another camera?
This information should be useful in picking cameras for photographing sports, birds, insects, race cars, et cetera.
Sadly no. Even worse, it is vastly unpredictable due to consistency at different focal-lengths, aperture and focus-distances. For DSLRs and SLDs, the choice of lens greatly matters too.
A number of review sites have methodologies and quote numbers in ms or fractions of a second which gives one relative measure but it not easy to generalize. In other words, you may get measurements saying that camera A is faster than B, but in another situation it is the contrary.
For compact cameras, the few that use Phase-Detection like the Fuji F300 EXR have the lead, followed by bright apertures ones like the Olympus XZ-1.
Among DSLRs, generally the ones with faster continuous drive usually have a fast AF and price to match.
Finally for SLDs, Sonly Alpha SLT use Phase-Detection. The Nikon 1 V1 and J1 also use Phase-Detection and claim to be extremely fast, although I have not tried them yet - one should be arriving soon.
There's no such standard.
As a rule of thumb, you should use a lens with moderately fast aperture (f/2 to f/2.8) for optimal AF speed - larger apertures will need more precision and smaller apertures are too dim. Try to pre-focus so that your focusing distance would be about in the ballpark of correct distance.
FWIW, my friend was recently searching for camera to shoot equestrian events, and after trial days with Pentax K-5, Nikon D7000, Canon 60D and Canon 7D chose the 7D with 70-200 f/4 lens based on their superior AF speed within his budget.
That would be next to impossible to establish, the number of factors involved is just too great.
Not only are environmental factors always different (light levels, distance, target size, speed, etc.) but cameras will behave differently with different battery charge, different lenses attached to them, maybe even depending on whether they're tripod mounted or not, what exact set of options and settings you've selected.
