I edited my OP and added the PopPhoto.com AF speed results for the current APS-C top dogs the CaNikon 60D and D7000 for easier comparison.
If I may make a comment -
The K-5 and both the K-x and K-7 AF at LV (Light Level) 6 or brighter are already faster than those CaNikon hotshots.
LV=6 is f/4, 1/4sec @ ISO100 so it is not that bright......
as PopPhoto describes it about the level of kitchen lighting....
But it's the low light levels that the Pentaxes seem to lag.....
I do a lot of very low available light photography with the K-x and two kit zoom lens (OK, stop laughing please?
).
I recently did the calculations - there are some areas at that venue where the light is so low it actually falls below the spec'd minimum levels of both the AF and metering.
I realize that one could do better allowing in more light through the lens - so I bought a Pentax-A 50 f/2 prime - it's not that great a lens since it was cheap - but it ought to be better than zooms, especially the cheapo kit zooms......
Well after a lot of re-practice to get my manual focusing right - I took it to my dark venue to use.
First thing I noticed is that I could not focus fast enough - remember it was dark, so hard to see through the VF - even if I had almost 2 stops advantage viewing through the lens.
However the metering was now just in range - so I could see the correct exposure - but it was the focusing - the green Hex was very useful but on real live subjects it was hard to get it stay on - whereas on either of the humble kit zooms the lock was really good - once I found an area to focus on - and on that night the difference was HUGE - I had almost no confidence of focus with the manual focus lens (and I have years of experience of manual focus on pre-AF film SLRs) -
Whereas I was actually delighted and even after over 20 sessions at that venue - surprised at what the K-x and either kit zooms could manage to focus on.
Now again I am not claiming world record breaking speeds - and of course I am now used to that venue and level of lighting and the K-x's performance - so I may have a better tolerance/acceptance level - but I would say that the K-x really does not lack for much in its low light AF performance for my use - and that is very low light with live musicians who constantly move.
Please see thread:
Kx in Use for lots of low light samples,
or go
directly to this most recent shot (and navigate
BACKwards - 418 shots)
The Pentax K-5 is a slight improvement over my K-x - so what's not to like?
Of course YMMV