Alright, testing with only one body is rather difficult, but I still did a quick and dirty test that hopefully will help you assess the situation, at least a little bit.
Camera : Pentax K-5ii
Focus mode : single center point
Light level : fairly dim, all indoor lightings, settings for proper exposure were about iso 2000-3200, f/2.8 at 1/125th of a second
Target : the side of a grey shoe box with big letters, about 4 meters away
Approximate results:
Times are in seconds. So, in this case, 0,51 means 51/100th of a second. I think it makes sense, I just used my editor (Final Cut) timeline to approximate the time.
*It failed to acquire focus, I had to repress the shutter, then it acquired focus. I repeated several time and it was just a lone incident.
video :
I'm sorry for the video. I tried to do something quickly that would allow for an "easy" visual comparison, but I quickly realized it would take me a lot of time to make it right, so I ended up doing something that I find pointless, but some of you may benefit from it, so I post it anyway. However, I figure you could just ask me for the original videos and you could then sync them yourself and maybe time them on a more precise fashion.
Finally :
While I had the camera set up, I figured I would test some other lenses that might be of interest for portrait work, or for general AF speed concerns.
Therefore, I have similar videos for the following lenses, all taken minutes from each other in the same light condition, at the same approximate distance of 4m :
Pentax DA*55mm f1.4
Pentax DA* 50-135mm f2.8
Pentax DA* 60-250mm f4
Pentax DA ltd 70mm f2.4
Pentax F 50mm f1.7
If you would like access to any of the videos to make your own comparison, just pm me, I'll send you a link for download.
I have not calculated the time for the other lenses, but I plan to do it eventually. The F 50mm 1.7 seems quite fast.