So I just ran some tests comparing my Red Sea Pro and Salifert KH test kits. The total time to test including adding water to the vial, drawing up reagents, running the test and rinsing out the vial was 1:16 for the Red Sea kit and 1:24 for the Salifert kit. The Red Sea was a bit faster, but hard to say that either takes an excessive amount of time. Neither of these have steps that require you to swirl for 15 or 30 seconds or otherwise wait like the Ca/Mg tests.
I ran 3 tests in succession with each kit. Of note, the first 3 runs with the Salifert kit gave very different results. I ran the test 2 more times, which matched the first result, so I think that is likely correct, but the variability was concerning. My testing methodology didn't change with any of the runs, so I don't have a good explanation for the variation. The only possibility I could come up with was a drop or 2 of tap water left in the vial that altered the results, but i don't think that happened.
On both of them I stopped titrating when no further color change was noted. I also noticed that the range from when the first transient color change occurred to when the color change was complete and stable was larger with the Salifert than with the Red Sea.
Regardless, the Red Sea results were pretty consistent and agreed with the Salifert results if you discard runs # 2 & 3. I did not have any sort of reference to check the absolute accuracy of either test. Based on these results, I would have to recommend the Red Sea Pro test kit. It seems to be more consistent, is slightly faster to run and I find the titration holder to be more convenient and easier to use than simply holding a vial in one hand and the dropper in the other.
Red Sea
KH (ml used)
8.7 (0.62)
8.7 (0.62)
8.8 (0.63)
Salifert
KH (mL remaining)
8.6 (0.44)
9.6 (0.38)
11.0 (0.29)
8.6 (0.44)
8.7 (0.43)