On the topic of the lag - I hadn't noticed that at all until reading this thread and then looking for it, that's how significant that issue is for me.
On the wider topic of K-3 Flash Issues here's another odd but marginally insignificant observation: When I have a legacy manual lens attached to the camera and one of the Pentax hot-shoe flash guns mounted and then switch the flash unit to wireless controller mode, the K-5ii will immediately disable this wireless mode on the flash gun and set it to P-TTL for me, but the K-3 will leave it indicating enabled, implying that everything is armed and ready, and then just not fire the flash. It kind of comes across as though the flash subroutines were not quite completed when the K-3 was released, or at least not as fully polished in some ways as the K-5ii. Better metering notwithstanding, of course.
As long as I don't make mistakes like leaving the ISO set too high, etc. I have found the flash exposures to be consistently accurate with the K-3.