Why would AF be so slow on RX1?

The RX1 uses contrast detection autofocus (CDAF).

According to Wikipedia, CDAF does not require the use of a separate sensor to assist on focusing, can be more flexible (as it is implemented in software) and potentially more accurate. But most of the time it is slower than phase detection autofocus (PDAF), and even more so in low light conditions.

I'm betting most reviewers saying the RX1 AF is slow is because they are used to PDAF on DSLRs. Steve Huff, on the other hand, is more familiar with small rangefinder style cameras and the Micro Four Third cameras that use CDAF, hence he is ok with the AF performance and finds the camera to be "pretty quick" (his actual words in his latest video). On his web site, he wrote "faster than the Fuji X cameras. About on par with the NEX-6."

If you look at the performance pages in the camera reviews at http://www.imaging-resource.com/, you will see most DSLRs full autofocus shutter lag is around 0.1x secs. Compare that to 0.2x secs on the Micro Four Third cameras such as the Olympus OM-D, PEN E-P3, Sony NEX7 or the Panasonic GX1. Now when Fuji launched the X100, the initial production firmware had a full autofocus shutter lag around 0.4x secs.

So, without an actual RX1 in my hands, just looking at what had been reported on the web so far, I believe the AF on the RX1, at firmwire version 1.0, performs about the same as leading Micro Four Third cameras and NEX. Given the RX1 has a sensor 4x the size of a M43 sensor and somtimes 2x the number of pixels, this is no small feat.

I'm pretty sure Sony will continue to improve the RX1's AF performance through firmware updates, just like it did on the NEX cameras. However, hoping the RX1 AF would be as fast as a PDAF based DSLR would be unrealistic.

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