having set thousands of posts of all kinds and sizes, i would offer this thought - its not the concrete speed or strength that matters, its the straightness of the post. and fyi, nobody who makes a living building fences mixes their concrete outside of the hole. they either use premix or they add powder to water right in the hole. it works just fine. and i live in ontario where cold weather cracking and frost heaving are a normal part of life.
if you use standard sakrete, you have about 30minutes of fairly fluid concrete. that means that you have to watch the post carefully for or brace (extra cost and labour). multiply that by 10 or 20 posts, and it gets expensive.
if you spring for the fast set, it sets up so much faster, that usually by the time you have started drilling the next post, the first one is stiff enough to leave alone.
they are both about 40Mpa right out of the bag, and even if you only get 20Mpa as the final strength (not a perfect mix), its more than strong enough for fence posts.
however, there are alternatives.
1) use hot water, and add directly to the hole. then pour standard sakrete powder in and mix with a rod. dont worry if its not perfect. its just post concrete. if your mix is off a little, it may result in powdery spots, but the moisture will cure it all within a couple of days. just try to get the mix proportions right when you do the addition of water.
2) just add calcium chloride or sodium chloride to the mix. it acts as a catalyst. just google it and you will see how it works. just dont use it for steel posts or anything with rebar. it corrodes uncoated steel quicker.
3) just add catalyst to the mix. its a little faster than bleach, but a little more costly. once again, see google.
4) this is the best way, and we have been doing it this way for about 4 years now - just use the expanding foam pouches. they are just as strong, unaffected by cold or ice, and weigh 1/1000 as much as the concrete. it costs almost exactly the same for the materials, and is far less labour. one guy can set 100 posts in a day and isnt tired at all. its so much more cost effective to use the foam that we havent set a fencepost in concrete for years now.